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.
Thank you for the feedback! Right now we're going to pull our leads from SalesForce. They have been collected over time from tradeshows, meetings, forms, etc. Since many of them may not have heard from us in months, if not years I didn't want to bombard them with a ton of emails right away.
It seems like 3-4 weeks is pretty normal with an increased follow up if they attend a webinar or fill out a form (which I already have triggers set for)