There are far too many variables to accurately predict that as it would really depend on how attachment happy your users are, and how much back and forth there is via the system between staff and client.  I mean I have users that attach 3 files per ticket.

The amount of text?  The number of replied?  The number of custom fields?I cannot answer your question (see previous answer)

I've got about 22k tickets in the database, with 1GB of attachments, total space used according to phpMyAdmin is 1.1GB.. Space is used roughly:1GiB for file_chunk (all the attachments, 18K rows of chunked data)50MiB for ticket_email_info (stores all message ids and headers, 53K rows)35MiB for ticket_thread (the messages back and forth, approx 53K rows)23MiB for session (can probably be pruned, approx 50K rows)7.2MiB for ticket (I have extra fields and indexes, with 22K rows)Then its below 5MiB rapidly decreasing to a few KiB per table.So, based on that, with no attachments.. roughly 176000 tickets should fit.. assuming no logs in /var/log etc..Edit: Just tested, yes, you can safely truncate the session table, it only saves staff sessions.