There's a lot of factors to consider.
You have to rely a lot on your team to wombo, MF doesn't seige or wave clear as well as a lot of other ADCs but she can position a really good ult and siege as a team. She's immobile with no form of self peel so she can't really one man carry the game and has to rely on the team engage+peel her. So the result can vary depending on what champs your team has, what champs the enemy has, how is everybody playing, builds etc.
If you want statistics though, here's her winrate at specific game lengths:
0-25 minutes: 53.03%
25-30 minutes: 50.76%
30-35 minutes: 53.19%
35-40 minutes: 44.86%
40+ minutes: 53.29%
Source: http://www.op.gg/champion/missfortune/statistics/adc