Surely the answer is (1) as much as possible but (2) realistically unless you're very senior that means you take what you are given and (3) it's all sort of made up anyway since it could be diluted so beware of over generous offers.
I guess it would help to know the current share price and what the same share would be worth at IPO. Usually you can "guesstemate" at that by:
* knowing the current revenue and costs
* guessing how that will grow over the next 4 years
* decide what the market's prevailing P/E ratio will be in 2-4 years assuming for the sector and country you are in to get to share price
* work out how diluted you think you will get
* applying some risk discount to all this
It's not a bad exercise to do to be honest. But every one of those numbers have a big range of uncertainty around them. You may end up with a range of share prices from 10-1000USD per share. Then you have to decide how many to buy but it's like lottery tickets after that...