A 10% participation order trades 100 shares for every 1,000 the market does, so completion time depends on how much the market trades rather than on the clock. That adaptability is the point: you finish sooner when liquidity is plentiful.
It is also the main input to any market-impact estimate. Under a square-root impact model, going from 5% to 20% participation roughly doubles the impact cost per share, which is why the rate is the key dial between urgency and cost.
A sensible ceiling for anything that wants to stay unnoticed is 5% to 10% of volume. Above 20% you are the market, your order is visible in the tape, and both impact and information leakage rise sharply.
Related: market-impact, execution-algorithm, capacity, vwap-algorithm