Effective April 15th 2015, only a single category of Tourist Visa will be available for US citizens the Indian Government will no longer offer 6 month or 5 Year visas, only a 10 year multiple entry tourist visa. The actual length of validity will be decided by the visa office on a case by case bases, some applictants will receive less than 10 year visa.

The Period of stay for each visit is maximum up to six months from the date of entry
Validity of a Tourist Visa commences from the date of issue.
Tourist Visas will generally be issued with multiple entry endorsement.