Spark client is the client provided on the same site as Openfire, but it is a desktop client.
I’m not familiar with WP8 clients, so i can’t say how will it work with that limit. You can only find out by testing. maybe start with 1 minute and go down gradually (50, 30, 10 seconds). If your client is sending keepalive packets to the server, then it should work. If it is not (this may impact battery life, as keepalive packets should be sent probably every second or more often), then your client will start to disconnect often.