Most lionesses will have reproduced by the time they are four years of age. Lions do not mate at any specific time of year, and the females are polyestrous.
A male and a female lion away from the pride are probably mating. They may mate hunderds of times in a three to four day period.
The success rate of matings is very low, only about one in three copulations results in cubs. This may serve to ensure that the lioness conceives to a healthy male and a good father. Males play an important role in protecting cubs from intruding males so for a female, the more persistant a male is, the greater likelyhood he'll be around until her cubs are grown.
As with other cats' penises, the male lion's penis has spines that point backward. During withdrawal of the penis, the spines rake the walls of the female's vagina, which may cause ovulation. However, this can also be painful for the female. This may be the reason why upon dismount the female will regularly take a swipe at the male as he flinches away.