They say you should never take the first offer, so expect to be offered something insultingly low.
Meanwhile, do some research and find some rough examples of just how much your car would have been worth based on model, trim, mileage, condition, age etc... Ignore the undervalued ones, though and get the best values you can find, obviously!
My bike was written off at £3,800 repairs, but valued at £900. I showed similar examples and got a professional valuation at £2,600, ending up with £1,800 cash after buying back the "wreckage" for about £360 in the end.
£160 for some 2nd hand parts and a new MoT to the insurers, and the bike was back on the road in a day. Has been awesome ever since - That was 4 years ago. So much for write-off!!