How long can you take the depo prevara shot?

Mt Grandmother has been taking the depo shot to regulate hormones when she began menopause. She has been taking it for around 2 years and wonders if she should stop taking it. The Dr. asked her to have another pap but my grandmother refuses bacause she is old and doesn't care to have one. She needs to know if it is unsafe to use it for many years in a row. Or if she has to take a break how long should the break be?