It's time for a rewrite when:

The cost of rewriting the application + maintaining the rewritten application is less than the cost of maintaining the current system over time.

Some factors that make maintaining the current one more expensive:

• The language is so old you have to pay people that know it a lot of money to program in it (COBOL).
• It has become so complex that the //Here be dragons. comment is all over your code.
• You can't write any other projects and add new value to the company because you're always patching this ugly beast.

It's time for a rewrite when:

The cost of rewriting the application + maintaining the rewritten application is less than the cost of maintaining the current system over time.

Some factors that make maintaining the current one more expensive:

• The language is so old you have to pay people that know it a lot of money to program in it (COBOL).
• (from experience, unfortunately) The system is on a hardware architecture that is so old that they have to scour Ebay and COLLECT parts to add to the machine it is running on because they aren't made anymore. This is called "hardware life support" and is expensive because as parts become more scarce, they (may) go up in price or they (absolutely) will eventually run out.
• It has become so complex that the //Here be dragons. comment is all over your code.
• You can't write any other projects and add new value to the company because you're always patching this ugly beast.