[Finnster]

Here's what I do to judge runtime and buy batts, I've got it from quite a bit of data using my ET and monitored batt charging.

For a "trackable" setup on a truggy or MT setup, I use roughly 5 Joules of energy per minute. I don't drive WOT everywhere, but works pretty well for hard bashing as well. Using that figure, you can gauge any setup independent of cell count.

However, to make things fair between cell types (A123s, lipo, even NIMh) I use loaded voltage estimates

EX:
Voltage*capacity (Ah)=Energy (in J)
For a 4S 5000 lipo pack:

4 cells*3.5V/cell*5Ah= 70 joules
so that: 70J / 5 J/min=14 mins
----One caveat---- You will not pull the entire pack capacity out. Realistic runtime is more like 12-13 mins. Also, if you drive harder, also less. Its prolly safer to overestimate a bit and use 5.5 J/min or so. Some track runs on my Revo I was only pulling 4.5 J/min tho, but again driving somewhat conservative.

For an HV setup on A123s (assuming loaded voltage of 2.8 V/cell):

10cells*2.8v/cell*2.3Ah=64 J
64 J/ 5.5 J/min =12 min
Pretty damn close to what I get @ a track pace:

So.. to answer your question:
6*3.5V*5Ah = 105J
105/5.5=~19 mins May clear a 20min main if you don't jam it hard.

If you want to find a pack to run say 7-10min on 6S
min: 7mins*5.5 j/min=39 Joules.... 39J/ (6*3.5V/cell)= 1.85Ah
===> A 6S 2100 pack should cut it, a 2500 pack would give a nicer cushion if you have a hot setup or drive really hard. 3700 would be overkill runtime wise. Most likely you will need a 25-30C 2500 pack to give enough burst amperage tho. A 30C 2100 pack may cut it. YMMV tho.