Realistically, a long, long time yet. Let's take the Soviet Union in WW2 as an example of fighting on despite horrendous casualties. If we look at only military deaths and not civilian ones, it lost about 5.4% of its 1940 population. If Ukraine were to sustain the same proportion of deaths today against its 2022 population, it'd add up to two million. Even Russian estimates of Ukrainian deaths are nowhere remotely close to that sort of number yet.

Obviously that'd be a truly horrific outcome. But in terms of the raw number of people available, Ukraine is a very long way from exhaustion.