Need help going back to brown

 I have had my hair an almost platinum blonde for the past two and a half years. I am now pregnant and dont want to bother with the upkeep and wont have time to with a newborn baby plus my other child to look after.lol. So years ago I had light blonde hair and got it professionally done and they filled it using a semi permanent dye with a red base and then paper toweled that off and then put the permanent color on. It held great. My natural color is a dark blonde to a light brown and I dont want to dye it that way myself just to have it fade away. How long would I leave a semi permanent hair color on before paper toweling it off ? Its been awhile since I had that done and cant remember how long it took before putting on the permanent. Hope this isnt too confusing. Can someone give me some tips on using a filler to keep the color from fading?