I have seen ears up as early as 6 weeks but it really varies. If the ears are up prior to teething (approx. at 4 months) then you should not have no worries. During teething, 4 months thru 6 months, the ears can go up and down quite a bit. After teething, the ears should then go back up.
My pup had one ear up at 3 months but the other ear struggled to come up. I waited until teething was over and the started using the ear forms to assist the other ear to get stronger.