How much battery does a 3DS use up on standby with streetpass on?

Solution 1:

According to this IGN Article, all versions the 3DS and the 2DS should have a battery life of about three days when in sleep mode.

Here is a simple chart comparing all the major differences between the three major Nintendo 3DS models:

    Battery Life    Nintendo 2DS     Nintendo 3DS XL     New Nintendo 3DS XL
    3DS gaming:     3.5-5.5 hours        3-5.5 hours         3.5-7 hours
    DS gaming:          5-9 hours         6-10 hours          7-12 hours
    Sleep Mode:           3 days             3 days              3 days 

You can find an archived copy of the original source here. It appears to have been published by an official Nintendo website, so it should be reliable.