How long will you hold on to trade-ables?

You can start by trading the ones for distance as soon as you can.
However I don't thing I will ever trade my legacy pokemons, except one I've kept from August 2016 to get a lucky Rayquaza.

I rarely hold onto special mons unless it’s something that is already pending someone waiting. They used to just stockpile up since you can only do one/day

It depends on your situation, how soon do you expect to be able to trade them, how badly do you want to free up the space and how easy will it be to get new tradeables if you start deleting the ones you have now?

I personally would not hold onto a lot of trade pokemon if I didn't have regular trading partners, but this is a very personal preference. At the very least I don't think it's logical to sit on replaceable IV rerolls or distance pokemon for more than a week or two, but your tolerance for them taking up your space may be different. This is a very personal preference I think.