
Here is a Kennel Club guide
http://www.the-kennel-club.org.uk/kennelclub/documents/13744Stud_final.pdfFrankly as the labrador is the most overbred breed in the country with 45000 pups registered with the kennel club last year alone and many other non registered litters born, unless your dog has some outstanding virtues that other top notch studs do not possess I would forget about having him used at stud.
The calibre of bitch owner you are likely to attract may not be very high, and you woudl not sish your boy to contribuyte to poorly bred or puppy farmed litters the members of whom are sold to anyone with the money and often once the novelty wears off end up in rescue in large numbers.
Morally you should be willingto help with the homing of nay pups that need rehoming sired by your dog, for their whole lives as in the best homes curcumstances can change.