how many frames should be filled on my second box before i put on the third box? i have an 8 frame. currently there are two frames unfull
General rule of thumb is when they've drawn 8 of 10 but in a super flow, heck, add that extra box. Also depends on your schedule. If you can't get over there to keep up with them better to add another as insurance then have them become honey bound.
...JP
In an 8 frame set up, if 6 are drawn out, it is certainly time to add new a box, especially if you are in the middle of a flow. In fact, if there was a strong flow going on I would add another box with only 5 drawn out. Rule is about 70% drawn out is time to add new box.( but just about all bee rules are made to be broken).