I think ti depends on what your goal is. Many of my clients will remove a prospect from the flow if sales has engaged with the lead (i.e. lead stauts changes).
If you are a freemium platform, you will want to do nurture emails to get folks to upgrade, so a few in the first four weeks. I believe we do 3-4 in the first month, and a different email if people become inactive after the first two weeks.
If you are following up with folks from an event, then maybe 2 per month for the first month, then 1 per month or every six weeks. All depends on what your goals are.
If you are purchasing a list and dumping people into a nurture campaign, make sure you are following can-spam compliance. I believe you can only email these people once, and if at that point they agree to receive your emails then you can continue to email them.