How long breeding season lasts? I have 2 males, one of them is chasing my female goldfish few months already, and she layed eggs just once (and ate them 3 hours later ).

Well, most spawning seasons last from early spring with the most activity. Then through summer it slows down somewhat, yet you can have occassional spawns, and sometimes it pops back up in fall when temperatures change again. But again, most spawns are done in spring.

Is your female bigger or smaller than the male? She might get exhausted if she is being bothered too much over a prolonged period of time...

She is bigger than the male; I think she was a little exhausted. Now she looks OK to me, because he's chasing her from time to time and I think she's carrying eggs, but chase is not so intensive. The other male is not interested, he joins sometimes, and they are very, very fast. First male is not only chasing her. He gets her in the corner and then pushes her with his head and body in the belly. Interesting thing to watch .

Yeah, it is, isn't it? Sometimes it can get really rough, especially with comets, commons, ryukin and oranda. The other breeds seem somewhat gentler, but not by much...