koi spawn when water hits about 65* in the spring. they wont spawn until theyre about 12" as a general rule. altho there are exceptions. I had one female who spawned at 8". my first spawn has always started the next morning after I moved them outside. (june). I use spawn mops and move the egg covered mops to a hatchery pond. spawning for me usually lasts a week to 10 days. most spawns are early morning 4am - 9am.. first day or two are heavy..then lighter off and on short spawns. that's been my experience, in Michigan. other climates may vary. I collect 100,000 eggs probably. not all are fertilized. many are gobbled up...its a fish eat fish world!