i change mine about every 2 weeks. i'll just top it off with a lil water and nutrients between changes. everything seems to be doing fine so far.

It really depends on what you're growing.
I've been known to run for a month or more without a change; just topping off with water and/or nutrient depending on the EC levels. With tomatoes, the fruits were about tennis ball size. maybe they'd have gotten bigger had I changed the solution, who knows?
I also grew some herbs in about 3 cups of water and never completely changed the solution, just adjusted it every now and then as above.
My systems are, however, DWC or a DWC/aero hybrid. with ebb n flo, you may want to leech the medium every now and again to prevent salt build up.

For what it's worth, I NEVER change my grow water. I closely monitor EC and pH, but I just leave the water alone.

To save your back, look into using a siphon tube. This can be any hose long enough to go from what you're draining to a toilet or other drain. Just fill the tubing with water; the easiest way is to submerge the whole thing under water. Put a finger over each end or keep each end at the same level and stick one end in whatever you're draining, while keeping your thumb or finger on the other end.

The idea is to keep ALL the water in the tube while you're moving it around and placing the ends in their respective locations; one into the source and the other into the drain.

Remembering that the destination end has to be LOWER in height than the source end, after placing one end of the tube into whatever it is you're attempting to drain (without letting any air into the tube while you're doing all this!) stick the destination end into whichever drain you choose and remove your thumb. The difference in atmospheric pressure will do all the work.

As soon as the source end begins to draw air, the "suction" will be lost and you'll have to bail the rest.

I have found an easy way, just add 2 valves and some extra hose to the pump and let the pump, pump it out.

You can also use the this to mix your nutes with.

Here is a link that might be useful:

I had the same (back) issue and decide to buy an additional cheap ($12) pump along with some 1/2 hose. I use to to either pump out the old nutes, pump in a new batch, or top off.I use an old surge protector so i can simply switch on and off the pump without having to pull the plug to stop the flow. probably the best investment i have made for general maintenance. As a byproduct, I always have an extra pump handy if my primary fails or becomes clogged and requires removal for cleaning.

You can get a electric siphon pump to do the work for you and save you back. Home Depot had a DuraHeat Battery Powered Siphon Pump for $12.95. I hope this helps with your nutrient changes, because it made my life a lot easier.

Here is a link that might be useful: DuraHeat Battery Powered Siphon Pump