In Dragon Ball Super Vegito defused quickly because he used too much power as super saiyan blue. IIRC potara fusion was supposed to last 1 hour, but for the power used it didnt. IIRC metamor fusion was supposed to last half an hour, but when Gogeta used it as super saiyan blue it didnt seem to have the too much power will defuse problem Vegito's fusion had. In Dragon Ball GT , Gogeta defused for using too much power, but since this series isnt canon, I dont think it counts. Also, how was all this for Gotenks, I dont remember if Gotenks ever defused for using too much power. Does Gogeta metamor fusion last longer than Vegito potara fusion?
This actually brings up an interesting point. Traditionally, a Potara fusion is supposed to last for 1 hour and the fusion dance is supposed to last for 30 minutes. However, just like you said, SSJB Vegito defuses in the Future Trunks Arc on account of being way too powerful.
- We do know that the potara fusion is stronger than the fusion dance. We do not know to what extent it is. However, Gogeta Blue is the Broly arc is a combination of a significantly stronger Super Saiyan Blue Goku and Vegeta, and I personally believe the potara fusion is not that stronger in comparison to the fusion dance(Note: The fusion dance also requires both the participants to be at equal power and Goku and Vegeta in their SSJB forms are equal at full power, unlike Goten and Trunks).
- Secondly, the potara fusion is something only accessible to the God's and it was stated that the fusion would last for an hour unlike the fusion dance which has a time constraint of 30 minutes. After rewatching the episode with the fight between Merged Zamasu and Vegito, when Goku and Vegeta defuse, Shin mentions that it hasn't been an hour yet. You can watch a clip of the same here.
- I understand the fight between the two fights lasted for a few minutes in the anime but in reality, DBS time can be as slow or as fast as possible based on the plot. The tournament of power was nowhere close to 48 minutes in real time and some fights which seemed really long literally lasted for a few minutes for the sake of plot. So, since Shin doesn't explicitly mention how long the fusion of Vegito lasted, it wouldn't be incorrect to say, maybe the fusion lasted even 55 minutes. Factor in the shocked expression on Gowasu's and Shin's face, maybe the fusion last for only 50% of the time i.e(30 Mins) or probably 40 mins? We don't know for sure.
- Yes! Dragon Ball GT Did have Gogeta defuse earlier than the 30-minute mark. However, like you said, the series is not canon to the main series and while there is evidence in Xenoverse 2, DBH etc with regard to Super Saiyan Blue being stronger than Super Saiyan 4, you still can't compare the two series with regard to this.
- Finally, the story is written by Toriyama and he loves to incorporate humor into the storyline. There is a possibility he kept the 30 minutes simply to include the failed version's of Gogeta's fusion to last for long while Frieza gets beaten up or something of a similar fashion which we won't know for sure until we watch the movie.
In conclusion, I personally feel the Gogeta Blue we see in the movie should be significantly powerful in comparison to the Vegito Blue from the Future Trunks arc. Also, the only reason Vegito Blue defused was obviously for the sake of plot and have Future Trunks defeat merged Zamasu. So technically, if the series progresses and they have Goku and Vegeta fuse again. It is certainly possible for them to have Vegito use something like Ultra Instinct Fused or Super Saiyan Blue + Kaioken*20 fused and last long enough for the fight to be interesting even if in reality, MUI is significantly stronger than SSJB. However, The most important fact here is that, it was never stated how long Vegito stayed fused as Shin only comments they did for less than an hour. So we have no way of knowing for sure.
You are misunderstanding the point yes vegetto while super saiyan blue lasted far less than gogeta at super saiyan blue you have to realize what they did in their fights when goku and vegeta had fused with the potara earrings vegetto had transformed straight into super saiyan blue and each and every hit was aimed to kill merged zamasu and he had used the rest of his energy in his final kamehameha however on the other hand gogeta had fought broly in his base form, his super saiyan form then his super saiyan blue form thus conserving energy plus super saiyan blue wasn't needed as broly was outclassed even super saiyan god would have been overkill but he went blue so that the gap was so huge that broly could not adapt and get strong enough to harm him he could have probably defeated broly while a super saiyan three and pretty easily the only way for broly to take on super saiyan blue gogeta was if he had transformed into a full powered super saiyan 4 aka a legendary super saiyan 4 we do not know how much gogeta was holding back as every form he was in gogeta had fought broly without any effort of struggle plus Gogeta was sparing his energy without vegeta knowing how to use the kaioken or goku knowing how to use super saiyan blue evolution neither Gogeta or vegetto could use them let alone combined the two and vegeta's nature is not compatible with ultra instinct so Gogeta and vegetto could never activate ultra instinct even if they met the same conditions as goku had against jiren