I largely use HHKB layout keyboards and although this doesn't really answer your question, moving from ISO to HHKB took me about 2 weeks to fully adjust. I still miss having the £ key though, so for any non-HHKB layout keyboards I still actively look at ISO if I can. I'll say that if you use an Apple keyboard (MacBook etc) you'll find they're already about halfway between ISO and ANSI (@ above 2, " above ' etc) which can give you some idea. At this point I can use ISO, HHKB or an Apple keyboard and I won't have to think about it at all really, I'll just naturally adapt to the layout.
I largely use HHKB layout keyboards and although this doesn't really answer your question, moving from ISO to HHKB took me about 2 weeks to fully adjust. I still miss having the £ key though, so for any non-HHKB layout keyboards I still actively look at ISO if I can. I'll say that if you use an Apple keyboard (MacBook etc) you'll find they're already about halfway between ISO and ANSI (@ above 2, " above ' etc) which can give you some idea. At this point I can use ISO, HHKB or an Apple keyboard and I won't have to think about it at all really, I'll just naturally adapt to the layout.