There is no actual limit but a creditor must reaffirm the debt exists at least every 6 years, there is no limit to how many times they do this but in reality they will probably sell the debt on to a debt collecting firm and they will do their best to collect. The debt never really dies if it is kept alive by the debt collector or creditor.
Not quite right Loosehead, a creditor has 6 years inwhich to collect on a debt. If after 6 years they have not had a payment or written confirmation from you in that time the debt is unenforceable. It will still exist but the creditor can no longer use the courts to get back the money and they cannot harrass you anymore. The debt becomes Statute barred.
If they creditor gets a CCJ against you in those 6 years then the debt is enforceable forever and the creditor can chase you for it forever.