Well it would depend but on average it would be from the day the child was born to 2-4 days after may be a bit longer it some cases but in my own research i found it to be only 2-3 days after.
Nope there is no shortage of marriages it just would depend on the amount of people of certain name in a parish to have 1-2 Bourke etc marriages a year would be about right and where you could have 10-15 Ryan or O'Brien marraiges as those family would have been the most common at the time.