Hello,
well, there is no easy answer on this.
What is the limit? What limit you mean?
Do you mean what is the limit of stored URLs in the _redirection table? Well, that greatly depends on your DB server, whether it is shared or dedicated, what is its HW configuration etc. Although MySQL has its limits, it is capable of handling hunderds of thousands or even millions of rows in one table. Still searching should be pretty fast thanks to indexing.
If you mean the limit of queries done during 1 request at your site, that is dependant on how complex your pages are (how many links have to be translated during one request). Here it is question of site design and this can also be optimized by caching. Having many DB request in one site page request can slow it down, but this is not really as much related to the number of records in table, more it depends on the total number of queries.
So I am really not sure, what would you like to hear. If you are considering other solution, what would that be? Of course, you could get a solution that does not use DB storage at all (as e.g. default Joomla! SEO does), but then you would not be able to get so \"nice\" URLs. So, more then a questoin of JoomSEF limit, this is a question of your server and DB limits. Considering JoomSEF is DB-based SEO solution, we made everything to make it as much effective as possible under given circumstances (way how Joomla! does work).
So there is a plenty of options, plenty of solutions. I do not know how many pages your site has, how many visitors per hour you have, how much new content is added every day., what is your HW, is it shared or dedicated... but even if I would know these, I wouldn't be able to say 500.000 is the limit -- that would need some benchmarking and checking conditions you have.
If your site seems to be slow even with 45.000 of records, then either the DB server does not have enough capacity or it is configured wrong. 45.000 of records in one table in DB world is \"nothing\".