dnd 5e – How can I balance the blessing of a weapon through prayer?

I am not an experienced player and I need help.

I'm playing a level 3 cleric (trick domain). The DM has stated that with a prayer session long enough in a sanctuary I can bless the weapons without using a spell.

However, the DM wants you to find out the effect of the blessing and how to balance it.

  • What would be the right time for a level 3 cleric
    pray to bless a weapon?
  • If the blessing lasts or several charges / attacks
    before it disappears?

The DM is also inexperienced and uses many house rules.