Someone had written up a nice explanation of how long they have been traveling from using the seasons and holidays in xxxHolic, but I have no idea where I read it. According to that, Watanuki is now 17/18 when he was originally around 16. They've gone from before valentines day early in the series to full blown winter with snow, and now they're in summer. There was also a New Year's in there as well, with the first dream of the year and all.
I need to go through and count the seasons and holidays, but it has been well over a year in Yuuko's world, that's for sure. You need to take the time difference into consideration when you're comparing Tsubasa to xxxHolic though.
Amongst the group, Fai and Kurogane have spent the longest amount of time together. Due to their seperation that one time, they lived together for six months longer than Syaoran and Sakura.