HOw long does it take to get a visa to enter into china as tourist in shangai beijin and Xi'an??

The normal time in most offices is 4 working days including the application and collection days. Of course there are quite a few holidays coming up so it might take more time altogether. There are also express services in many offices but it varies between offices. Also there is an extra fee. For opening dates search 'China visa mexico' and see if you can find the relevant Chinese website, where it usually has this info.