Upvote:6
I haven't been able to find precise information, but one source does seem to indicate that the baptism occurred in the Williams' first year, as per the norm to be expected.
The 1874 book, The Conqueror and His Companions, Volume 1 By James Robinson PlanchΓ©, has a section going over some of the arguments concerning when William was actually born. This bit mentions a source (which I have not been able to locate) , a location and a year (emphasis mine):
3 The cartulary recently discovered at Falaise recording William's birth and baptism there in 1027
This indicates William was both born and baptized in the same year (1027), in the town of Falaise. (If anyone comes across a copy (or location) of this cartulary, please drop a link in a comment)