you need to give yourself at least 10 minutes...without walking away from your screens. it depends how many shares you want to buy versus how liquid the stock usually is. assumng you want to buy 5000 shares, and the stock can move on 2500 shares, I would do this :


if the market is 20/20.5, then put in a limit order for half your amount at 20.5 (don't show your whole hand). whatever is not filled within 1 minute, cancel it. if the market is higher and you are not filled on your 2500, then think about whether or not you really want the stock, if you really want it (actually, you should've thought about this beforehand), pay the offer again for the balance of the 1st half...if you get filled on the first 2500, and the market is unchanged, then there's a higher chance the stock is going down than up in the very short term (this of course assumes the assumptions above that the stock can move on 2500 shares...this is not INTC we're talking about)...and then wait 10 minutes and execute the balance of the other half....if you had gotten filled on your original 2500 and it is still offered at the same level, there's a good chance you will get a lower price on the next 2500 ... if you had to chase to fill your 2500, then you give them a little breathing room ... they can't see you, 10 minutes may be enough for them to think you aren't coming back for more, and thus you get a higher chance the stock is back to a fair price rather than them waiting to rip your head off.