I agree the trial need to be at least two weeks before the event so it will have faded off fully. She won't wnat to have snake skin on the wedding day.
Reasons for the trial? Well I can think of a few, and if you put yourself in the brides shoes you will probably think of more.
Firstly they want the right solution and shade of tan,
They want to know if they have any adverse reaction to the solution,
They want to see if you do a good job, they also want to know how patient and personable you are ( make sure they like you)
It is a stressful time for a bride to be and they want people that don't add to the stress. They want to know how long it takes to apply the tan, how the fade off is as they don't want to look yukky on their honeymoon.
I try to do the trials close to the hen night, that way I can maybe get some of the bridesmaids or guests to have a tan at the same time, They have a nice tan for the night out and the girls can compare their tans and the fade off over the next week or two.
Try to convince her that the groom needs a light shade of tan on at least his top half for the big day, they can look washed out next to a glowing bride, especially if there are a lot of black and white photographs, I usually charge £35.00 for bride and groom tan, if they want a trial and tan for the both of them I charge £65. That way they are making a saving but at least I am still covering my travel time and fuel costs as well as earning something out of it.
I charge £35 for bride trial and tan.
Hope this helps