Too long
I'd say it's a way too long, get's boring after a 6 wave, and there is more.
The concept is nice, but the gameplay itself - is not thinked of carefully.
Either you'll blast at the beginning, due to wrong strategy, either you'll get bored by the 6 level - if you make it right - at that time no enemy, except capital ships, can get close to your turrets to damage them significaly. And the one capital ship - blows up before it can blow up my turret, so I heal it with stacked up in advance, while there were no capital ships, "heals".
Good game - need some polyshing to the gameplay.