There's this girl I really like, I told her about my feelings towards her and she said that she'll have to think about it because she just got out of relationship, so we just have to give it some time.
How long should I wait, and what to say?

Is this a high school dating thing or what?

?? You do not "tell someone your feelings about them" sounds more Jr High. You ask them out, you go on a date, if they like you, they go out with you. If you both have good time, you go on another date and talk or chat some most days.

Then over time, feelngs come out. You just do not "tell someone your feelings"