I think, ultimately, it will depends on how big a run they are doing, which I haven't seen announced so far.
If the run is as big as most previous (new series) releases, then you're likely to have plenty of time to grab him, particularly if you don't care about the 'extras'. If the run is closer to the recent Hartnell release (500) then they might sell out much quicker, given how many people missed out or passed on the first Smith release, and came on board later with the likes of Tennant, given the new series wider popularity, and also given the Capaldi head factor, which definitely widens the appeal, even to many who might have been perfectly happy with the first Smith release. The Capaldi head is supposedly limited to being 'while stocks last' as well, so who knows exactly what that means, but that might encourage people to order sooner rather than later as well, just to make sure they get that second head. Just have to wait and see what the numbers actually are, and how it all plays out I guess.
I'm ordering anyway, so way I figure it is that I may as well make sure I avoid any potential hassles and order right away.