Trying to concieve after depo? Too early for positive test?

I am ttc after depo. I got 2 shots and did not go for my 3rd. It has now been 3 months since I was due to go for my 3rd and I have yet to get my period back. I have spotted 2 or 3 times but not a full period. I was given a blood pregnancy test about 3 weeks ago and it came back negative, We have been trying since then but since I have no period I dont know when Im ovulating to better pinpoint. I took a 2 test within the past week but I think if I am pregnant its too early to tell. How long should I wait to test again if if im not pregnay know how often should I test?