There's no minimum wait imposed by law — a creditor could, in theory, chase the day after the due date. What actually varies is how much courtesy is worth extending before assuming the delay is more than a simple oversight.
The benchmarks most businesses use: 3 to 7 working days for a first, neutral reminder; 10 to 15 days of silence before a firmer second chase that states what the delay is already costing; 30 days of total unanswered delay before a letter before action stops being premature. None of this is written into legislation — it's accumulated practice, and the one rule that does matter is procedural: never let silence sit without a dated, written follow-up, whatever the exact timing.
