I don't want to be a spoilsport, but... Avocado seedlings grow really fast and tall, so it may seem like it's doing well, but eventually, it's going to reach the next stage in it's growth when it'll want to be a *tree*. At that point, water and, if you're giving it, water-based chemical nutrients alone isn't going to be enough. You'll also find that it's going to get top-heavy and start toppling over.
I've seen photos of avocado seedlings growing until the roots completely fill the water jar, with comparatively HUGE top-growth. But think about this -- as long as it's growing in water, it's growing water-roots, not soil-roots (they're different). If at some point, you decide that you DO want to plant your tree in a container, it's going to have to adapt and grow soil-roots -- and it won't be able to support the large top growth with the wrong roots. You can't keep the avocado in soggy soil during transition because the soil-roots of avocados require WELL DRAINED soil. In addition, Water-roots are typically thicker and brittle, and depending on the kind of jar you have and how much of it is filled with roots, it'll be difficult to extricate. Finally, these roots will be wound around itself and you won't be able to straighten them to spread out. This pre-disposes the roots to strangle itself, making it even harder for the tree to survive the transition and grow well.
If, on the other hand, you just want to grow this avocado in the water as a novelty, then by all means keep it in the jar.
There! You did ask. Trust me, I've been there.
Oh, if do want to pot up this tree, you should cut 1/3 to 1/2 of the growth -- sounds drastic but it WILL produce side shoots once it starts growing again (look for a good spot in the trunk/stem with a lot of buds). When it's first transplanted into the container mix, it just can't support all that leaf surface. It'll concentrate on growing roots at first, so don't expect to see a lot of growth. Also, it probably won't do much growing until the weather warms up again.