Hi Charlie,
The same question has been posted today on the French-speaking forum. Here is the answer (better late than never ).
The time limit is set by the admin, for the whole site, here:
Administration ► Security ► Site policies
Maximum time to edit posts // maxeditingtime
Default: 30 minutes
This specifies the amount of time people have to re-edit forum postings, glossary comments etc. Usually 30 minutes is a good value.