I base mine on the paint schemes post-steam for my New England roads. For example, post-steam to pre-Guilford on my MEC, B&M and D&H. I am guessing that the time frame is 30-35 years, but it varies as I include connecting roads. For the CN everything is pre-North American scheme and for the CP nothing after the pac-man. This does two things for me. I get to run a few versions of paint schemes that would have overlapped as the roads repainted and it limits my spending on new locos and rolling stock.