If I had to guess I'd say that one extra move for White (normal chess) is objectively a draw, 3 extra moves is probably a win, and 2 extra moves is a tossup.
The problem is, for anything short of 4 extra moves, there's no way to definitively prove any of the above results. Yes, Stockfish gives some evaluation that's much better for White, but unless it can calculate a forced mate then there's no absolute certainty.
There's also the issue that engines prune the search tree a lot in order to be fast. So when an engine says it's at "depth 30", it's not actually covering every variation up to 30 ply that has a chance of affecting the evaluation. Doing that would take very long, even with optimizations such as alpha-beta pruning and a transposition table. So technically if you're searching for a guaranteed win, you couldn't even trust an engine that says "mate in 50".
We can look at it more theoretically. In chess the average number of possible moves in a given position is 30, but to be very generous let's use 20 (because maybe White can launch a mating attack early on). And let's assume that with 3 extra moves, with a ton of luck White might have a forced win in 40 ply. So to calculate everything 40 ply ahead, this would be 20^40 positions. Let's assume alpha-beta pruning turns the exponent into around 3/4 of what it was: 20^30. Now, let's say the transposition table increases the speed by a factor of around 5 (that might be slightly generous). So we're left with 20^29.46..., or about 2.1 * 10^38.
So in the best case scenario of White actually having a forced win with 3 extra moves (and it only taking 40 ply), maybe you'd need to calculate something on the order of 10^38 positions to be definitively sure. This is obviously an extremely rough estimate, as the actual number might be a lot lower (again, in the best case scenario). For example, if White is trying to mate Black, the branching factor could be lower due to Black having only moves at certain times. Also, you could try only considering the best few moves for White in each position. However, it's likely Stockfish is already doing pruning somewhat similar to this for White and Black, and so far it finds nothing after depth 40 for a few different positions I set up with 3 extra moves.
You could argue that a win might exist in something much shorter, such as 30 ply instead of 40 ply. I think this is very unlikely though, considering Stockfish gives an evaluation of +1 and change, when it's over depth 40. Sure, like I said it could absolutely be missing something, but I'm not using Stockfish for a proof here. I'm just mentioning it to say that a very quick win likely doesn't exist, and then if that's the case it will be near impossible to prove a longer one does due to the discussed time complexity.