I did it a few years ago in early February. France was extremely easy, cars almost lined up - toll booths work great. Spain got more difficult the further inland I got, with the worst being attempting to get out of Toledo. The first time I tried it was on the edge of downtown, which failed miserably after four hours of waiting in the slushy rainy snow - I was waiting in a really bad location though. What worked the second day was taking the local bus to the real edge of town, i.e. the edge of the suburbs. The waits in Spain probably averaged 45 min, but it was winter and people were probably in bad antisocial moods anyway. I'm sure if you give yourself a little time buffer you'll get where you want to go.