I've heard 10-20% dilution each round (assuming the company is doing well), and probably 3-6 rounds before liquidity or indefinite illiquidity. Obviously there are circumstances outside of that, but that already ranges from barely any cumulative dilution to having a third of you original stake.

If the company isn't doing well, then dilution can be worse, and your valuation will also be lower, so you will own a smaller piece of a smaller pie.