98% of the work I perform, and anything done in the past 5 years working with other companies, have not been contract. It is almost, always time and material work. The clients understand that retro wiring isn't straight forward and there can be obstacles to deal with. We always give them an estimate, and we are very close to that, but that is due to more experience than anything.
For pre-wiring, if it is on contract, then I take what time is needed to complete the job. It is sort of hard to screw up pre-wiring if you have done them for a while.
I can pre-wire a 4000 sq/ft house in one day. It all comes down to planning and having everything you need with you.
Always plan where you want your drops, box the house, drill the holes for the wire runs, place all wire at the panel location and pull to all the boxes. the more wire boxes you have, the more runs you can place and the faster you can wire a house. I usually have 4 boxes of rg6, 4 boxes of cat5e, 2 boxes of 16/4 speaker wire and 2 boxes of 16/2 speaker wire.