17 seconds most probably will be frustrating for your players.
The proper way to do things would be to do playtests - ask some strangers to play the game for a few minutes while you watch. Also, play the game yourself! What is YOUR average life time in your game?
It is important to playtest with unbiased people - your friends or family might say they like the game even if its terrible.
You should make notes. Does the player know how to interact with the game? Does he make mistakes? What makes him frustrated? What does he seem to enjoy?
Only after doing your own observations you may ask the player for an opinion, but it usually wont be as useful as your own observations. You as a game designer should have a better understanding of how games work and utilise a broader vocabulary of game related terms to verbalise the design problems your players encounter.
There is no single mobile game session time. Some people will play it at for three minutes, waiting for a bus, others will play for half an hour while commuting. The game should be designed with a specific playtime in mind, this should not be left out. The game should be designed to trigger some emotions and have a conclusion in the specified playtime. You cant do that in 17 seconds. Your game probably gets too hard to fast, making people quit after the first or second try.