We sometimes get questions from people who don't have any hearts to give and can't understand why. Basic members get 1 heart to give in any 24 hour period. If they give a heart at 7 pm on one day, they will have to wait 24 hours until their heart is available again to gift.
It gets a little more complicated for paying members who have 5 hearts* available. It does work the same way as for members with just one heart. If they give their 5 hearts out one an hour from 6 pm until 10 pm (i.e. at 6, 7, 8, 9, and 10 pm), they will have used up their allocation until 6 pm the next day. At that time they will have one heart available. This will increase to two hearts at 7 pm (if they don't use the available heart in the meantime!) and so on until they have 5 hearts at 10 pm. The cycle then starts again.
If you don't use all of your hearts in a 24 hour period you don't get any more after another 24 hours. You will only ever have a maximum of 5 hearts* available.
*In some cases for certain contributors to the crowd fund an extra heart (or two) may have been added to your allocation to reflect the rewards provided.