I have some that are reallylong haired.
The young in the litter from these these two longhaired and a SH banded fawn are very longhaired but I cna't get any pics of them at the moment as they are like 'Zebedee'!!!!!!
I'll try and get some pics in a more normal stance but these are very jumpy mice. I dont' know if it's becuase they are argente or not????
With these the long hair is apparent by about the 14-16th day.
Some of my 'not so longhaired' can take a little longer to notice though.
I've foudn the litters are not as strong as the normal litters so I take them down to about 4-5 at the stage I can tell they have long hair. This gives the longhaired pups a better chance of growing to a reasonable size.
I do the same with the astrex and texel. Unfortunately you have to wait a little longer to cull than you would normally but it makes for better pups. If you keep an entire litter they are usually runty little things.