Whenever debt service reaches 25% of the budget seems like it would be the kick off point. Though that's more than half a guess.
I've looked into it a little, and the information is as usual polarized. In it's simplest form, inflation is a wonderful method of paying off old debt. The Germans cracked that code after WW2. Wasn't much fun for the general population, but worked out well for the country.
Sometimes debt is a sensible and necessary option, but a never ending pile of debt will absolutely have repercussions. Those repercussions can be mitigated by using an advanced management system called "kicking the can down the road", but like everything else, it has limitations.