'Standard' business practice is 28-30 days and should be included on your invoice if not otherwise arranged, if you do not receive payment within this window then usually a reminder after 7-14 days overdue and a second a further 14 days for immediate payment, this can be different with large companies who pay on 60 -90 days but that is your choice to take the work on.
If you do not follow this standard practice then you must pre-arrange payment terms with your customer, failure to do so does not favour you legally or help your reputation.
I get where your coming from, suffered the hard times, slow payers etc so have full sympathy but you need to ensure everyone is aware of your terms, if this is already the case then I fully am on your side here.