It's designed to vest at the same rate that the employee adds value. For the sake of simplicity it's almost always set up to be four years with a one year cliff.

i.e. at month 11, the employee has no options, at the end of month 12 they have 25% of their options and then from there on in the options vest at the end of each month so that at month 18 they have 37.5% of their options. At the end of 4 years they have the right to buy all their options.

It can vary depending on the employee and what they negotiate upfront but unless they are bringing something that is of particular one-off value (a rolodex of clients for instance) there are not many reasons to change the standard 4+1 format.