there is no definate answer to how long im afraid, could be a couple of weeks, could be months, so many factors eg how quick the third party fills in forms etc
As Len said, i would never take the first offer. If you have not got an agreed value then i would get examples of what similar vehicles to yours are selling for within the scene/enthusiast prices and send them back when you reject the offer as the insurer is likly to only offer their idea of "book" price which can be shocking.
Only down side though is that means it will take even longer!
best of luck!