The answer is… it depends.
It depends on things like:
Your marketplace, your prospect list, your relationship with your prospects, how aggressive you want to be seen, how much you need new clients etc.
In saying that, most MSPs will typically benefit from either a weekly or bi-weekly cadence in sending Nurturing Emails to their Prospects.
The emails are written to be non-obtrusive and conversation starting so anyone that Unsubscribes from them is highly likely not a good prospect for you and you should celebrate them Unsubscribing as they’ll be saving you money in your CRM costs 
In our Monthly Marketing Packs, we give you 8 different emails (including subject lines) that you can choose from for that month.
If you really don’t like the thought of sending Weekly or Bi-Weekly Nurture emails, then at a minimum, consider sending out an electronic version of the Printed Newsletter that we create for you each month.
You can find an email template for how to send it on the bottom of this FAQ post.