Hello! I think that the function you asked for is not implemented in Photon, but it‘s already suggested: Dashboard button groups
Another idea for you now: You can set the buttons in flash buttons, so can send the Midi signal for the whole 15 Minutes and after it you deactive the signal and activate the new one.
Another thing about the tine in sequences: you can set the time higher than the fader allows. Press in the field with the number of seconds or bpms and enter a higher number, 900 seconds (15 minutes) worked in my case.