Generally, the longer your co-founder remains a stakeholder, the longer he/she is going to be an 'asset' to your company, even if they are not actively involved in the company.

At the same time, not allowing them to cash in for a very long time, could backfire as they may become frustated and start being a pain.

A good balance might be to have a staggered vesting period for different percentages. example: if their total stake is 35%, then let them divest, in increments of 5% (max) each year, starting from end of year 2.

End of the day, you have to figure out what is the best and a reasonable configuration (for both parties) that will sustain your company while rewarding those early contributors.