There are no set a rules but there are some they go by. However I saw rate went up after so long in DOM. What do you say about this? There are lot of different factors per case.
Rate negotiation is available at any time but usually longer DOM makes better chance.
Your good buyer agent will help you do that.
In real estate anything is negotiable and it doesn't hurt to try...but of course you must consider a few factors that will better leverage you in your negotiations, like you mentioned the longer the property has been on the market the less desirable it looks to other buyer or tenants. This will give you more of an advantage when negotiating new terms and conditions. My advice to you...go for it! Best of luck!