There are a lot of answers about how, but the more important question is why you want this information. If you are trying to figure out the market trends on how long it takes for a home to sell, then a sample of one might not be a good indicator. There are many reasons why a home may be sitting on the market longer than average and several of them relate to mistakes that you would not want to make in selling your own home.
Conversely, if you are wondering about on market time in preparation for making an offer to buy, then your best bet would be to ask the sellers directly. The length of time on market does make a difference, but other factors can be even more telling about whether you can offer a lower price and these would be best determined based on the specfics of the situation for that home seller. Elements such as why they are moving, if there are any upcoming timelines that impact motivation etc.