How long does it take for a guppy to have fry?

Provided her conditions are suitable 28 Days. If conditions are not suitable she can hold her young until she dies rather than give birth. Guppies need a temperature of 73F to 75F and you will need to follow the rules . :- 1 inch of fish needs at least 1 gallon of water but more is better. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week.Follow the rules and you stand a chance of keeping fish successfully. Miss out on any of them, and I can guarantee that your fish will have constant health problems.