There's only one sure way - You need to test it yourself.
Estimation result largely depends on your actual code (how efficient is it) and shaders (how complicated are they).
Also, there's number of parameters you need to take into account, each can dramatically change the estimation:
- Antialiasing (ranged from Off to 16x ?)
- Display resolution (0.5k to 4k)
- Anisotropic filtering (ranged from Off to 32x ?)
- other GPU settings (performance vs. quality slider)
- slow CPU can also be a bottleneck
EDIT: Generally you need a bunch of testers, who will play your Alpha/Beta and from whom you collect performance data and decide if you need to reduce polycount or not. Also it is generally a good idea to add Options slider to allow players to tradeoff picture quality for speed. Some prefer higher FPS, some prefer picture quality.