It seems like an hour or so for me maybe a little longer. So when I worked out on the Carnival Cruise over the weekend I went straight from the gym to the pool there were some awesome hotties so I needed to look my best.
You dudes have got to go on a Carribean cruise. WOW
My pumps usually last 60-120 minutes after my workout. If you take a fast acting protein source with a easily digesting carbohydrate source after your workout, it'll increase muscle glycogen and amino acid uptake in the muscle and extend the duration your pump lasts.
After each workout I take 30g whey isolate (I use isolation from n2bm.com) + 30-80g waxy maize carbohydrate.
It depends on how much glucose you have in your system and if you trained properly. Never though in terms of maintaining a peak pump to impress the ladies though. If you look good, you look good. If not, a pump ain't gonna help much.