The reactions we give(Likes , hearts, facepalms...) are limited in use to 40 in a rolling 24 hour period if you use them all up you will be out until you get to the last time you used one. Lets say you use one at 7:45 am and then you use the other 39 before the end of the day( before 7:45am the next day) you would have to wait until 7:45 am the next morning for that one reaction to become available for use again. As each new time you last used a reaction another one becomes available for use.
I never run out of Love but I do run out of Hearts