Need help with buggy conversion

Oh, what sort of runtimes are you guys getting with your setups? I haven't settled on a particular pack configuration yet. I'm looking for that perfect compromise between weight and runtime. I was going to go with 4Ah 2S packs, but now I'm thinking that I might want to go with at least 6Ah 2S2P pakcs. I'd like to be able to run for 20 minutes. My track is going to 15 minute mains and the electrics will be required to make an obligatory pit stop, but I'd rather just make the obligatory stop and just sit there rather than actually HAVE to change batteries.