15-20 seconds of idle with a decent throttle response is good. there are no awards for letting an engine idle for a whole tank. Eventually the thing will load up with fuel if you have the top end rich enough to be safe and lean enough to make good power. You have to rev it up every now and then to clean it out. Then it will idle another 15-20 seconds or so. You should never have to wait longer than that for the start of a race, and if you do, have your pit guy pick up your car so you can clean it out, or drive it around with some quick bursts of throttle and line-up again.