Ovulation kits say that once it turns positive you will most likely ovulate within the next 12-36hours....and there for if you have had sex the previous couple days and while ovulating you will have you're best chance to fall pregnant...they even say to start trying to concieve 3-5 days prior to ovulation and then a couple days after for the best possible chance...they say some sperm can last up to 3-5days so i think it just really depends...