It might last that long, depending on how often you hit from it. It wouldn't last me 8 hours, but others have said it lasted that long for them, or longer. The Riva is the same as an Ego, so
eGo Manual Battery (1000 mAh)
This will last almost twice the time of the original battery, which was 650 mAh. I also got tired of the charging all the time thing, but I went for a boxmod which uses a 14500 900 mAh battery (size and shape of AA battery). The
batteries are charged on a normal Li-ion battery charger, which one can get for about $12, and they are much cheaper
batteries, and the boxmod itself is light and durable.
Box Mod
Has a bit higher voltage than the Riva too (3.7 working volts vs 3.2V working). Fits in the pocket nicely, has a master on-off switch so it doesn't go off in the pocket, and one can throw a couple extra batteries in the pocket or bookbag or purse if you use one up.
The Riva is very popular here, but it might be a bit weak with a Boge carto...it was to me when I tried that combo. I believe there are adapters now for the Riva/eGo batteries which allow the use of regular 510 battery chargers.