It would be hard to overdose on it per se, but I wouldn't take much more than 3 tbsp a day...that's 13.5g of SF.
The problem with SFSs comes if you increase the dose too much too fast...then it can cause gas and bloating until your body adjusts. That's why Heather suggests starting on a tiny dose like 1/2 (or even 1/4) tsp twice a day. A dose like this won't help much, but by increasing the dose slowly you can avoid all side effects.
I was put on a SFS before I discovered Heather VV and went straight onto the full dose. I had major gas and bloating for a couple of weeks, but it worked really fast...it just depends whether you want to avoid the side effects or not.