Like Tithing, a "generous fast offering" is between the Latter-day Saint and the Lord. President Kimball said,
"Sometimes we have been a bit penurious and figured that we had for breakfast one egg and that cost so many cents and then we give that to the Lord. I think that when we are affluent, as many of us are, that we ought to be very, very generous... I think we should be very generous and give, instead of the amount we saved by our two meals of fasting, perhaps much, much more—ten times more where we are in a position to do it. I know there are some who couldn't.”
(Conference Report, Apr. 1974, p. 184 - This quote won't be found in the General Conference files online, as it was part of an address to the Welfare Services Meeting on Saturday)
Don't fret about how much you give. Give until the Spirit tells you it's enough, and that will be different for every person, every family.