How long after a man ejaculates can he go at it again? Is it 20 min. for a guy to create more sperm?

It takes longer than that to produce sperm. What comes out in ejaculation is a mixture of sperm and semen. The sperm is produced in the testes and the semen is produced in the prostrate. Most of what comes out is semen which carries the sperm along. When a healthy guy ejaculates he doesn't let go of all this semen so if he's healthy enough to get another erection he probably can come again in 10 - 20 minutes or so for the 2nd time and longer for the next, etc. However, there'll also be less and less sperm in there because it takes 24 hours for a full recovery of everything. A healthy guy can probably have an orgasm fairly often in a day but the liquid will soon run out... and so will his energy level.

I can keep it up after I ejaculate if need be and then do it again within several minutes without missing a beat.

That depends on the guy, but for the most part, everything being equal, he can ejaculate again as soon as he can get an erection, and that depends on the amount of stimulation he gets after the first ejaculation, and how aroused he is by his partner. There are times when a guy can maintain the same erection after he comes, and come again in a matter of minutes.

Most young guys can come 3,or 4 times or maybe even more in an evening, especially if he is with a 'new" partner and she is somewhat experienced. It could be 20 minutes or even less between ejaculations.

For me, I can usually have sex again after about 1/2 hour.

I’m not sure if you’re asking how long it takes before we can have sex agen or rather how long before we can ejaculate agen. Every guy would be different. It takes me less then a minute to get another hard on but the second time around takes about an hour to ejaculate.

about 30mins

i would say for me its 3 to 5 mins really depends on the guy...