@Roy Oliphant
I was under the impression that for vacant land not designated for agriculture use and non-homestead residences the redemption period is 6 months. Perhaps what I am missing is a separate right to "contest" the sale itself, which may be the 2 year contestability period for all property you are mentioning? Are they two separate items?
http://comptroller.texas.gov/taxinfo/proptax/tc06/ch34b.htm#34.21
Title 1. Property Tax Code, Subtitle E. Collections and Delinquency
Sec 34.21 (e)
(e) The owner of real property sold at a tax sale other than property that was used as the residence homestead of the owner or that was land designated for agricultural use when the suit or the application for the warrant was filed, or that is a mineral interest, may redeem the property in the same manner and by paying the same amounts as prescribed by Subsection (a), (b), (c), or (d), as applicable, except that:
(1) the owner's right of redemption may be exercised not later than the 180th day following the date on which the purchaser's or taxing unit's deed is filed for record