When it is a legitimate visa for a legitimate reason, they generally don't harass you at consulates. It is only overuse of tourist visas by people who are not really tourists.
I had year long ED visas previously when it was permitted years ago, then they 'cracked down' on it and reduced it to 90 day only with possible extensions. It also used to be allowed for 3 years or more with a yearly extension, now it is only 90 days with 2 extension up to 9 months (as far as I am aware). Please make sure you know exactly what sort of visa you have Helen so you can share the correct information with people on this group :)
If you have a visa stamped and extended every 90 days then you have a 90 day visa that can be extended. You do not have a 8 month, 9 month or any other longer term visa. If for any reason they decided to NOT stamp your visa then you would be going home. There are only a few long term visas available for people staying in Thailand and you don't have one. As Robert says you have a single entry ED visa that is extended.
So send the application and passport to the embassy in Canberra along with an express envelope to get it back. It is not really that difficult to do, just takes a bit of planning and getting the required documents together.
Nope no limit.....'officially'.....until an Immigration Officer has a bad day and pulls you up on it. Also I recommend staying away from Female agents.