I couldn't find anything on the internet and there is also nothing about this in the cluebook or manual.

Level | XP needed for next level
1 | 100
2 | 300
3 | 600
4 | 1000
5 | 1500
6 | 2100
7 | 2800
8 | 3600
9 | 4500
10 | 5500
11 | 6600
12 | 6600
.
.
.

The formula is just current level times 100 + the XP needed for the current level. This goes up to level 11 but then stays at 6600.

I know XP grinding isn't really rewarded in this game because one only gets 2 skill points per level and there is a better way to get this points but perhaps someone might find it helpful. :)