What's a reasonable amount of time should a person wait for someone to love them if you already love them?

-In most relationship there is always one person truly love before the other..I'm just trying to figure out when is it a good time to see that it probably wont happen without wasting too much time in a dead end relationship!It could take a second or a lifetime... I can assure you their is no LIMIT...... it just a feeling of attraction..... like a magnet if both attract.... him & her were made for each other ;)
Be open about your feelings..maybe he feels the same way and isjust waitng for you to say somthing?
I will say 3 months top.