A number of people have asked for this feature.
It is coming to the android in the next weekish, but you can do it now with the python tool. Here’s an example:
You can also use this tool to set any of the device parameters which are stored in persistent storage. For instance, here’s how to set the device to keep the bluetooth link alive for eight hours (any usage of the bluetooth protcol from your phone will reset this timer)
pip3 install --upgrade meshtastic meshtastic --setpref wait_bluetooth_secs 28800 Connected to radio... Setting preference wait_bluetooth_secs to 28800 Writing modified preferences to device...