One item that should be born in mind is that Jomres can be very resource hungry. It has been optimised as much as possible to generate as few database queries as possible by preloading a lot of data. This is particularly true in the Property List and Booking Engine functionality. We achieve this by running a handful of queries and storing this information in arrays.
This allows us to run a lot of complex code for example in the booking form very very quickly. This means that data that needs to be regularly looped through is instantly available, rather than pulling it from the database but it does mean that Jomres can have very high memory requirements and it is not unheard of to get out of memory errors from PHP. Whilst it used to be a problem in the past, it's less so nowadays.
The default 8Mb that most PHP builds have available isn't enough. Joomla itself needs almost all of this 8Mb simply to run and adding Jomres on the top will definately mean that you will run out of memory.
What to change
If you're suffering from out of memory errors, you can try editing your php.ini if you have access to it, and raise the limit a bit like so :
Note that you may need to try different memory limits until the problem disappears.
Alternatively, if your hosting service allows you to, you may also be able to use a ".htaccess" file to raise the limit:
''php_value memory_limit 20M''
Other modules and plugins (mambots) in Joomla and Wordpress can contribute to out of memory problems simply due to their existance. If at all possible, if's a good idea to try to have a minimum number of modules showing on the same pages as the Jomres application itself.
Two other Joomla plugins can greatly contribute to the memory consumption in your Joomla install: Joomfish and SEF (particularly sh404SEF). Joomfish to a lesser degree, but to a greater degree sh404SEF. This isn't by the way a criticism of either of these plugins, they're great applications in their own right. Rather, I raise the point to illustrate to the site manager (you) that these other plugins may bring their own set of problems, and that you need to be aware of them.
We've tried to write Jomres to be as fast as possible. As mentioned above, we've achieved this by trying to run as few mysql queries as possible, however even the fastest servers will start to buckle at the knees if you've got a lot of properties.
On our test servers its is very fast with a few hundred properties listed, however once we got to over 10,000 properties things started to slow down (although it was still functional). We would suggest that in it's current incarnation 10,000 properties might be the upper limit on a fairly standard dedicated server. Shared servers will be much slower as you will not have exclusive rights to the server's resources.
Note that these tests were done with Jomres installed on Joomla 2.5, Joomla 3.x is faster, and later versions of PHP have better performance too so your mileage may vary.