For me 18 months would be the upper limit and I'd prefer to start weaning at around 12 months.
I'm not into the idea of really extended breastfeeding (5/6 years) because personally I would be a bit disturbed if I had any concsious memories of sucking my mothers boobs but thats just me. Whatever works for you is best.
I would tandem feed if I had a second baby before I had weaned the previous one. It would probably be a bit of a relief when your milk came in to have an older baby to help empty your boobs. As for the colestorum I geuss that I would have a supply of EBM for when I was in hospital so it might not be an issue.