Every strain has its own specific harvest time, some sativas take as long as 3 or 4 months to mature and some indicas are done as fast as 6 weeks! If you know what strain it is do some research on the net to find its specific OUTDOOR flowering time, indoor flowering times are totally different unless you live right next to the equator. If you dont know what strain it is its gonna be kinda tough if you dont have much access to the plants. If its some commercial or mid grade the seeds were probably caused by a late flowering male whch means your buds will probably take awhile. In that case just wait until your first major frost and then cut em down. Now if the seeds came from dankity dank that just happened ta have a couple seeds in it they were more than likely caused by a female flower that grew male pollen sacks on her (hermaphrodite). In that case Again there's really no way to know flowering times unless you know the strain, I would wait til first frost. Either way if you wait to long past first frost your risking losing your harvest to the cold because you dont know whether or not the strain is resistent to cold temps. Another thing to consider is whether you like an up head high or a sleepycouch lock body stone. If you like the up head high then harvest early a little before the frost hits. If you like the body stone harvest after the first frost.
Just some grower ta grower advice, next year order some seeds from the internet (attitude seedbank is very reliable). That way youll know when to harvest and you can pick a really potent strain that will grow good in your specific climate. Also get some good organic potting soil and some additives like guanos, worm castings, compost etc. to add to the soil at your grow location. If you just plan a little bit and do some minor preperations you will end up with one hell of a harvest! If your gonna take the time and risk to grow definitely put in a little extra effort cause it will make a huge difference in the finished product and that just makes it so much more worth your time.
Happy growing my friend
PS A lot of info and people out there will make growing pot seem alot harder than it is, learn from every grow and do some reading and youll have great results