BTW, I did a shmup for Android last year and decided to remove the spaceship altogether because I found it a little bit awkward to move it with a finger:
http://bigosaur.com/blog/23days
I did it in just 23 days by sticking to simple pixelated graphics for the aliens and only drawing UI and powerups. I also borrowed music with CreativeCommons license and only created the sfx myself. This gave me time to add various boss levels which is what makes the game interesting (and takes time as you wrote).
Also, powerups can be upgraded between games, so this got me a lot longer playing time than 6 minutes. You play to pass as many levels possible, but also to collect coins to upgrade your weapons which in turn enable you to pass more levels the next time and so on. There are 14 different bosses and you can hardly beat bosses 9+ without upgraded weapons.
There was a good ExtraCredits episode on YouTube explaining game loops (I can't find it now, but I recommend watching all EC episodes to any game developer). I didn't know about it at the time, but Drone Invaders fits great:
- smallest loop: kill enemies in a wave, reload guns
- bigger loop: every 10 waves there is a new boss level
- even bigger loop: when you die, upgrade your weapons and try to go even further
- biggest loop: reach the Fleet Admiral rank but hitting certain score, number of destroyed enemies, level reached and hiscore achieved over lifetime of playing.
I an monitoring the stats (the game sends anonymous usage info at the end of each play), and there are some players playing the game for over a month.