Well, we know that the rogue gets:
12 + CON Score at Level 1.
5 hp per level thereafter.
If we assume that the 120 hp on a 15th-level fighter is accurate, we have to figure out some values that are reasonably in range of the rogue, and reasonably produce 120 hp.
Essentially, this is an algebra equation (call it A)...with too many variables...
A) X + CON + 14Y = 120
Assume, for a moment, that our Level 15 fighter would have a Con of 16. That reduces Equation A to the following (we'll call it Equation B):
B) X + 14Y = 104
Now we just need to take a guess at some values for Y.
If Y = 5, then 14 Y = 70, and X = 34. Which seems kinda high, but maybe.
If Y = 6, then 14 Y = 84, and X = 20. Which seems reasonable.
If Y = 7, then 14 Y = 98, and X = 6. Which is obviously crap.
Now, if you take a lower number for the Con, say 14 (odd numbers don't give good results), you get X = 36, or 22. The first is possible, but seems WAY high to me. Besides, the fighter should get more than the thief as they go up in level. And 22 is just a weird number. So, in my opinion, the fighter will get:
Hit Points at 1st Level: 20 + Constitution score
Hit Points per Level Gained: 6
And, if you want me to make a whacky guess, the wizard will get:
Hit Points at 1st Level: 8 + Constitution score
Hit Points per Level Gained: 4