He's around or over 50 days so about 7 weeks (depending on how far the
permanent dentition is in) because that's when the adult canines come in
according to James Fox's vet texts.  The baby ones should drop out soon but
if they don't your vet can remove them very easily with just forceps unless
they get wedged in which can produce real abscess problems.  A double set
for a short time isn't unusual and no problem.  Here are some other adult
tooth TYPICAL eruption times to help you narrow his birthday range down:
about day 53 (about 7 and 1/2 weeks) for upper molar 1; then at about day
60 (around 8 and 1/2 weeks all of the upper premolars and lower Premolar 2
(a placement designation ancestrally and ferret ancestors lost P1 form
their line-up); further along at about age 67 days (about 9 and 1/2 weeks)
lower P3; and finally at about age 74 days (about 10 and 1/2 weeks) lower
P4 and lower Molar 2.  Bob Church had a post maybe last year (year before?)
telling when the incisors tend to erupt, common variations, and the actual
ranges if you want to look that up.  OH, and Cheweasels really help reduce
teething discomfort; if you can't get them locally call the Ferret Store at
1-888-8FERRET.
[Posted in FML issue 3000]