So how common is it for a mangaka to maintain the same character design through out several of their manga's? Or is this just something Hiro Mashima does ?
In Japanese it's called Star system
Japanese version of wikipedia has a page for Star system of manga/anime
Manga/anime section describe 3 categories.
Same character, Same world. Negima, UQ Holder and CLAMP's manga are categorized to this. Each story is happen in same world, but focus to different time or character.
That Wikipedia page also mentioned about game. For example, In addition to 2d action, Mario is protagonist of cart game, tennis game and other.
It's definitely not something that just Hiro Mashima does; in fact, it's very common. Roel van Uden mentions Milk Morinaga in the comments. Sp0T mentions Ken Akamatsu. If you look back at Akamatsu's work, he almost always creates spiritual successors for his characters that have very similar appearances and personalities, e.g. AI Love You's Cindy became Love Hina's Naru who became Negima's Asuna; AI's Forty-chan became Love Hina's Kaolla Su who somewhat became Negima's Ku Fei; Love Hina's Shinobu became Negima's Nodoka Miyazaki; Love Hina's Kitsune became Negima's Kazumi Asakura. In the early sketches for Negima that are given as bonus material in the US manga release, you can see that the original design for Negi looked a lot like AI's Forty-kun with glasses. There are many other examples in Akamatsu.
As another example, in Kozue Amano's Amanchu, the character of Hikari is very similar in appearance, personality, and name to the character of Akari in Aria. (Both names mean "light".) There are various similar-looking characters across the CLAMP works, e.g. Sakura Kinomoto's older brother Touya resembles Subaru Sumeragi from X, and both somewhat resemble Shizuka Doumeki from xxxHolic and Rikuo from Legal Drug.
I think user2435 has a point that artists develop certain habits and tendencies that express themselves in the character design. But it could also be about staying recognizable; when I see the Negima volumes in the store, I know right away that it's by Ken Akamatsu, because Naru's twin-tailed little sister is staring at me from the front cover. Even if an artist is capable of drawing in a very different style, it isn't necessarily advantageous to do so. It may also be relevant that manga-ka tend to write the same kinds of stories for each work, and so might draw in the same style to make sure the art and story match. By the time we get to UQ Holder, Akamatsu has switched from love comedy to adventure, and there are subtle differences in the art as a result (Yukihime looks much more mature than any major heroine in his previous work.) CLAMP uses a slightly different style between xxxHolic and Chobits, even though the two works came out around the same time, because the stories are different.
In short, it's very common for manga-ka to reuse character designs, or to slightly tweak existing character designs.