I will be starting on a 4 bore around the end of the year. Two years out is really not very bad at all. I think most good builders will usually take longer than that. It might take well over a year just to get a barrel. If you want one faster than a couple years you probably have to build it yourself.
I believe Mike Brooks recently said he was out over five years if you wanted him to build a gun. Agree with billraby that two years is pretty quick and doubt you will find any of the top builders doing one any faster.