After getting an offer, what's a reasonable amount of time that I have to decide whether to accept or reject? What's the upper limit on this reasonableness?
And how many different (current) members on the team is it reasonable to ask to speak with?
On the topic of reasonable time before you decide on an offer, I just wanted to mention -- you don't need to (you shouldn't) immediately accept the deadlines from the company.
When I was negotiating between 6 offers in 2017, I was transparent about my process and asked for several additional weeks. The Big Tech companies were surprisingly ok with this, so you should just ask. Phrase it something like this:
This is a big decision and I want to ensure I can commit completely to the job I take on. I don't want my employer or me to have buyer's remorse, so I'd like to make the decision with all the data on hand. I'm confident I'll be able to do that within the next 2.5 weeks -- does that timeline work for you?
After getting an offer, what's a reasonable amount of time that I have to decide whether to accept or reject? What's the upper limit on this reasonableness?
The standard is 1-2 weeks. Anything above 2 weeks is quite generous as it gives you more leverage to get more offers and shop around.
However, a lot of companies will do "exploding" offers which expire in just a few days. These offers tend to be more competitive, but sometimes the company is just being mean.
I imagine the "lifespan" of offers will get shorter the worse the economy is as interview candidates have far less leverage, so maybe I'm wrong on the 1-2 weeks thing.
And how many different (current) members on the team is it reasonable to ask to speak with?
I expect something like this:
I recommend these resources about team selection as well: