I don't know that their is an easy answer to this in Texas with the obvious climate difference. Up here in Iowa we can get a decent stand of
switch on the 1st year by frost seeding onto previously killed sod and controlling weeds with atrazine. The frost seeding allows the seed to stratify through the freeze thaw cycle which causes a much greater germination the 1st year. The atrazine(which has no effect on switchgrass) holds back all other cool season growth allowing the switch to take hold. It is also important to kill all competition the year before with a couple doses of round-up and possibly a burning to clean up the trash.
When switch is seeded later in the year and weeds are not controlled with herbicide it generally takes 3 years to get a stand around here.