My girl is greasy no matter the season and needs a bath about every 5 days but I wait it out to 7 in the summer and about 10 - 12 days in the winter. I find that her skin does get dryer in the winter even though her hair will be greasy.
I will throw a towel in the dryer and holler to DH when we are almost done so he can bring it in to us on the very cold days of winter. Yes she IS spoiled