how many days off should a salary employee has right to?

In general, an employee has no right to a day off.  That said, certain industries such as airlines and trucking have regulations about how long certain employees can work.  Safety/OSHA might be a concern as well, but that discussion is too fact intensive for this forum.  An employee does have the right to be paid for all hours worked and any "on-call" time where he is not free to pursue other activities.  If you are properly classified as Salaried/Exempt (from overtime pay) you may just need to find another job if this schedule is not acceptable.  However, you might want to visit with either a labor and employment attorney or the US Department of Labor, Wage/Hour Division (there is one in each major city in Texas) to see if you have been properly classified as salaried or whether you should be hourly and be paid overtime (for hours in excess of 40 each week) and whether your "on-call"  time should be counted as hours worked.