Seems to me, that you can choose everytime that you do you "obedience"...
Remember you have to take that Deific Obbedience feat to make the
boons work.
"Benefit(s): Each deity requires a different daily obedience, but all obediences take no more than 1 hour per day to perform. Once you've performed the obedience, you gain the benefit of a special ability or resistance as indicated in the "Obedience" entry for the god to whom you performed the obedience.
If you have at least 12 Hit Dice, you also gain the first boon granted by your deity upon undertaking your obedience. If you have at least 16 Hit Dice, you also gain the deity's second boon. If you have 20 Hit Dice or more, you also gain the deity's third boon. Unless a specific duration or number of uses per day is listed, a boon's effects are constant."