I don't think there is a specific execution time limit but for obvious reasons on shared hosting you can't be eating all resources of the server to the detrement of other peoples accounts.
So as far as I know you can use the fair amount of resources that are allocated to you without a specific execution time limit.
I have used scripts such as Joomla with no problems on shared hosting in the past (as well as many other scripts involving PHP/MySQL.
Someone from eUKhost will obviously see this post soon and correct me if I am wrong or feel free to double check with them by directly contacting them.
Hope that helps and welcome to the forum community!
