What stock, exchange, and time of day?
If you're talking about MSFT, which trades 50M shares ($1.4B) a day, there's almost always 2K shares ($54K) available at the inside market, meaning no slippage for a market order.
If you're talking about B, which trades 234K shares ($11M) a day, most of the trades are a couple hundred shares, as is the inside quote, you have to be more picky about when you place your orders and should definitely use limit orders (though you might miss the market and not get filled).
If you're talking about AMO, which trades <10K shares ($170K) most days, you'll need to work hard and spend a lot of time to get in/out unless you see an obvious counter-party for your trade.