If you've experimented with wait times between 5 and 30 minutes, and haven't determined an optimal time for yourself, you may be experiencing some condition that could require medical advise.
As long as you are taking enemas that aren't harsh or of an extreme volume or temperature, you should be able to repeat them every 30 minutes or less.
I find that a soapy enema will cleanse my system and permit another tap water enema of larger volume within 20-30 minutes. If time permits, I can take additional enemas every 20-30 minutes until I have accomplished the level of cleansing I desire.
I have taken repeated enemas over a period of about an hour to simulate a colonic. Each enema gradually exceeded the volume of the previous one until I was filling my bowel to the cecum. I then would finish with a smaller cold water enema.