The rules of thumb (for dry cures) are suppose to be 7 days per inch of thickness or 2 days per pound. (Some say 3 days per pound for especially thick items; some say 4 days per inch for thinner items.) (Brine cures usually take less time; much less if injected.)
I don't find that either of these rules work out across the board at all. I think it's best to go with the recommended time per the recipe you are using, or from advice from others who have done what you are wanting to do, or, if creating something new, then going with a time based on a successful recipe that uses a similar cut of similar thickness and make-up, and similar curing agent quantity.
I wouldn't have a problem with extending a butt cure--though you could go 10 days, rinse, and hold in the fridge. For loin I wouldn't go 14 days either. I might see 10 if the cure was a dry one and the loin was not just the typical eye (as for Canadian bacon), but also had the other muscles still attached; for the eye alone, no. I also do not think that lean cuts (like loin eye; maybe even whole loin) do well cured, rinsed, then held. I think it is better to cure then smoke pretty much immediately.
