10 years :o
Pros:
Feels like you're getting a massive privilege to go back to baby/toddler land after so long. That wistful baby smell is so so delicious after being used to big massive upper primary school kid!
They are so different you end up really appreciating the cute innocence of the toddler and the independence/adult-ish conversation of the older one.
Older one can (somewhat) babysit - wouldn't leave him in charge yet but he can occupy the younger while I cook etc.
No toddler jealousy/aggression towards newborn, he was old enough to get it.
You already have the long-term perspective of the big one so don't stress so much over milestones, food, sleep, toddler behaviour, breastfeeding etc because it's obvious these things don't matter in time.
Although they are not at the same stage, this isn't as bad as everyone seems to think it is to manage - 12yo is still young enough to love having an excuse to go and mess around in sand/water tables at the park or gambol around soft play (when allowed!) despite the fact he would be much too cool to do these things normally. And the toddler is young enough to tag along to wherever, and not be bored because he's entertained pretty much anywhere as long as he can wander around and explore. OK, we're not doing family holidays at Peppa Pig World but quite frankly I don't particularly care! Plus, 12yo is perfectly capable of getting on a bus (corona excepted) and going to hang out with his own friends, or staying at home and having a fortnite marathon if he doesn't fancy an activity. Likewise, 2yo isn't aware enough to understand he's missing out if one of us takes the older one to do something grown up.
Cons:
We had got rid of everything so had to buy all new. Even after DS2 was born I haven't kept stuff of DS1's except a couple of sentimental bits, I don't want to be using 10 year old stuff nor storing it forever, it's not practical so you do end up buying twice. Things he used for longer (brio trains, lego, books) we have kept hold of, but not clothes, tiny kid toys or baby equipment.
Some of my knowledge is a bit rusty/out of date which can make you feel like a first timer again! Or just get into arguments on MN :o
Although they adore each other they don't have that really close relationship that I had with my sister, that I associate with siblings and had hoped for with my kids.
Their timetables clash quite significantly. I am getting woken all night because I'm one of those hopeless mums who breastfeeds them until they stop asking and then I'm exhausted in the evening which is when my 12yo has run out of screen time and wants to hang out. I think we could probably have more fun hanging out in the evenings if I wasn't so knackered and run ragged from trying to wear out a toddler all day! Preteens don't wake up until lunchtime unless forced to, so if we do family activities together we have to wake him up or we have a short window in the afternoon which suits everyone.