Really depends on your needs. We have a small leisure battery and it can go for about 3 days without a charge. We just added a split charge relay and as we drive every day when we are away that will be enough for us to run the lights, charge mobile phones and tablets and run the water pump... which is all we have fitted
Agree with comments about using 12v where possible, for an inverter think about what you want to run off it and don't go too much bigger than you actually need and I would say go for pure sine wave although they are more expensive.
Don't forget that the inverter can take a lot out of your batteries depending what you are running off it, amps from your battery is roughly watts divided by volts, for example 1200 watts from the inverter would take 100 amps from the battery plus a bit more for efficiency losses.