International is a pain, but as a general rule of thumb I would apply pressure immediately the invoice is overdue. The deal is you supply, and they pay. You’ve done your bit, so there should be no bashfulness about asking for your due.
In my company we put people on stop the day after due date, and will not supply further goods until the account is up to date. A county court application made out in their name appears through their inbox after about 10 – 15 days. Everyone told me that we would go bust if we took that attitude. Strangely, we haven’t and our payment situation is extremely good. We also pay all of our suppliers ontime, largely because we don’t have cashflow problems generated by slow payment of our invoices.