my 12/25/10 stamp allowed me to stay til 3/19/11; re-enter 3/22/11; leave 5/4/11, re-enter 7/31, leave again 9/14/11 re-enter 10/6/11 obtaining only 5 days, expired today (whoops, yesterday). Probably doesnt answer your question. But you can see there is variability, possibly due to the fronterras. also, i believe when i asked in january at immigration, i was told its january to january. thats been a while.
Feb 02, 2011
90-day Bolivia tourist visa duration by: Anonymous
You get 90 days every 365 days beginning on the date you enter the country. It's not a calendar year.