Pretending to sleep on Ambien is likely to lead to actual sleep very very quickly lol. At least if the OP is benzo intolerant, which is probably the only way a trip will be achieved to begin with.
As far for long it lasts, I never kept track exactly, but it's pretty short, definitely not longer than a few hours.... You want to avoid falling asleep at all costs if you're trying to avoid getting caught though. Sleepwalking is actually pretty common on Ambien and will seem a lot more suspicious than you just seeming really sleepy and out of it, at least if you don't sleepwalk normally.