Sometimes you can not go by how long it has been on the market . The seller may not be too flexible if they have not reduced the price.
First make sure you have signed a buyer agency contract with the agent. The agent will guide you and show you all the recent sales. You can base your price on recent sales . You do not want to pay more than what it is worth.
I was working with buyers recently who made an offer on a house that was listed at 565k. We offered 450 and came upto 475k.The seller countered at 520 . My buyers were willing to go upto 510, if it appraised so we ordered an appraisal .It appraised at 505 so my buyers decided not to buy and the seller was not willing to lower his price from 520k Your agent will guide you.