Invoicing clients

I'm presently doing some casual compliance work as a tax agent and am gradually accruing a reasonable number of clients. Most of the cases have been rather straightforward sole trader SA returns etc with a relatively small fee attached.

Having never worked as a self employed individual before, I'm unsure as to what time period to allow for clients to settle their respective bills upon completion of such work? (I've arbitrarily settled upon 14 days from the invoice date, but have encountered quite some difficult securing payment in this time).

Any guidance would be much appreciated.

Thank you.