How to dye fabric with tea?

I have an old burlap sack that I am wanting to incorporate into a quilt top, but it is stained, so I was thinking that if I tea-dyed it, it might mask the stains a bit. I read several how-to articles online, but the one thing I am confused on is how long you should leave the fabric in the tea? Most said "until you achieve the color you want", but approximately how long does that take, are we talking minutes or hours? Please help?