IMHO It's a decent size tank, however it's not ideal. I would recommend changing the plants to live at some point, as that would help with the amount of water changes.
I'm no expert but at this time you should change the water every other day, but if you have a lot of live plants and a filter you MIGHT get away with it once a week.
However, if you don't have a heater I strongly recommend yout get one.
Would you keep a dog in a small box? Then why would you keep a fish in a bowl?