Hey Bigbob, welcome.
There are many e-cig options that would last even a heavy vapor a whole day. Many are what are called "mods", in that they have batteries that you can change out as opposed to sealed batteries that are one-piece. Mods can get a little confusing for beginners, so I will postulate the following recommendations.
The
kGo is probably the most bang-for-your-buck sealed battery available. It is 1100mAh, & a decent rule of thumb is ~1 hour for every 100mAh of capacity.
Tons of things influence run time, like the resistance of the atomizer/cartomizer used on the battery (lower the resistance = more current drawn), the amount you vape, etc. Still, the kGo has a huge capacity & offers great power.
The
e-Power is a very popular mod, and comes with 2 batteries that are 1050mAh, which is a wash capacity-wise versus the kGo. The nice thing about the e-Power is the batteries are less expensive to replace than buying a whole new sealed battery.
Either would work well for you. There are even larger batteries for mods, which is a whole 'nother ballpark. We can elaborate on that if you're don't mind spending a little more, as these mods can range anywhere from $25 to $400, and involve a lot of fun stuff like chargers, adapters, extension tubes, etc.
Since you sound like you want something that is low maintenance throughout the day, you could look into something like this
tank. 3.5ml is a whole LOT of juice, and you'll probably get at least a day before needing refilling, probably much more.
I'm sure others will chime in with great recommendations as well as juice, but this is a start. Again, welcome!