In anything else that very liquid US and European futures or stocks, estimate about 10% to 20% of your intraday simulation profits to go to slippage. Short term position trading I would say about 5% to 10%. trend following you don't care, isn't that nice?
Some strategies do not suffer that much from slippage. Example, limit orders for entry and exit plus buying at the ask and selling at the bid.
Market orders, especially at the open of a bar to enter and at the close to exit manage to kill very good strategies in real trading. It is amazing to see the wide divergences between simulation - even with slippage estimation - and actual results.
Do not even think to trade Asian futures in high frequency. They are not stupid there. Some guy with a turbine will slip your market order ticket enough ticks to eat up all your profit.
Slippage is a real enemy.