I did it about 32 weeks, but I'm a bit obsessive with cleaning and organising at the best of times lol!
It doesn't take too long though as while there may be lots of clothes and blankets they're all pretty small, so if you've got lots of similar colours - eg lots of white and lemon etc - you can get quite a lot in one wash and dry it pretty quickly.
I did all the newborn and bedding, then cracked onto the 0-3. It only took about 3 days to get the lot done (before I was on maternity so just doing washes at night) and we've got a lot of stuff really as had lots of hand-me-downs given to us. I washed all the soft toys/play mat etc too.
I even ironed the clothes so they looked nice in the wardobe lol.. but I know I won't be bothering to iron stuff once baby arrives lol!