Short answer: When your server gets slow.

A bit longer: The advantage of flat files is that they might (I never did a benchmark) be faster, depending on size and the server's internal throughput on the one side and the database on the other side.
If you configure your server the way it keeps the user-flatfile cached the flat-file should be faster.

You also said only a few scripts use the database file, try to decrease the number once more. If you really don't want to hack into SQL you can also try to split up your textfile, seldom used stuff like a user-bio can be moved into seperate files (for each user one file, or maybe for all users starting with the same letter -> ~36 files) or much better into a static file which gets updated every hour/day.

There are various ways to get around SQL, personally I prefer SQL (cuz I don't have to pay for the database space) but I also like the text files (like in my diary) because I can update them a lot more comfortably.
It depends on the way you wanna do it and on your server 

--
package Lizard::King;
sub can { do { 'anything'} };

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In reply to Re: Text Handling Limit by TomK32
in thread Text Handling Limit by Anonymous Monk

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