About 6-7 years to pay off the stadium. The rest of the time they were just a poorly run club. Sadly they seem to be getting their act together, and quite importantly, backing their manager.
That's the thing though. For years they didn't pay off the stadium (still haven't). They just paid off the interest as it was so low they didn't think it worthwhile to actually pay off the bulk. What they did do was hoard cash, (think at one point they had as much cash in the bank as the rest of the league combined). This could have been for a couple of reasons that were never made clear. Arsenal were being used as collateral for the la rams building their stadium. Or wenger getting a bonus on player trading (making profits). They had to have cash reserves due to their agreements with the banks to secure the loans. Or a mixture. But they could have paid off the stadium a lot earlier but chose not too.