A lot of depends on the pricing in your area. For me, I just can't justify the several hundred dollars I am used to. Some of my new quilting ladies since I've moved have marvelous affordable quilters.
The other criteria I use is when you just aren't satisfied with what you can do yourself. I have done queen and even King sized quilts on my vintage machine but I don't want to do that anymore. I would usually do a basic grid of some sort on the bed/center of the quilt and then something fancier on the borders where I could work it. My fancy dancy modern machine with the extra large through has died and is no longer an available option.
I'm still hoping/planning on getting a long arm for myself. I am productive enough and have a backlog of 20+ tops all with backings and bindings, everything done but the quilting. I'd rather put in my money to a system I can afford than to someone else's skills -- but it is tempting!
The FMQ comes with practice. Nothing wrong with getting some sheets and/or blankets from the thrift store for practice on dog sized projects (basically crib sized) especially if you can find a rescue group that wants them. If you have the option, I'd recommend buying a roll of the batting you like/want to do FMQ on and use that but a lot of it is just practice practice practice and eye/hand coordination.