How many times do i have to wash my hair after relaxer?

My mom says its good to wash my wash every two days for about a month after a perm.But i wanted to get sew in extensions and if i get that i'll leave it in for like 3 weeks is that ok not washing my hair for 3 weeks?I've already washed it twice before this as i put it on monday.How many times do i have to wash my hair after relaxer?
After a relaxer, I usually wash my hair once a week. But if you plan on getting sew in extensions, you need to wash you hair before you get them, and after you get the extensions in go to your stylist once a week so that they can wash it and keep it looking nice. Don't try to wash your extensions yourself because it doesn't look right, at all.