How quickly can a guy get another erection after ejaculating?
I have been with a couple of guys who have almost no lag time between erections. Is this something that every guy should be capable of or have I just been lucky?
You have been lucky.
What you refer to as "lag time" is sometimes called the refractory period. This term is used to describe the amount of time it takes for a guy to become erect again or the amount of time it takes for him to ejaculate again. Sometimes the term is applied to female bodies and their ability to be touched after an orgasm or the time it takes for them to experience another orgasm.
One of the biggest factors in the male refractory period is age. The average 18-year-old male has a refractory period of about 15 minutes. Once a man gets into his 60s or 70s, that period is extended to about 20 hours. Some guys will exceed these averages, while others experience hypersensitivity in the penis after an orgasm, making any touch uncomfortable or even painful for a period of time.
If a guy is looking to reduce his refractory period, he may be able to do so by spending some time training his body. This is usually done alone. He will touch himself until he is very erect. At the point where he is just about to orgasm he stops the stimulation until the intense feeling has passed. He will want to repeat the cycle a few times. In doing so he is allowing his capillaries to expand and his ability to stay hard longer and to spring back expands as well. This practice is sometimes called edging. Edging may or may not have the desired effect ... but you never know until you try!