Battery life/performance depend on lots of variables. With a reasonably new/healthy battery, yes, it should hold a near-100% charge for a few days — but not weeks and weeks.
Don't store it in the car glove compartment, for example — low overnight temperatures affect performance. Turning it on & off repeatedly uses up a lot more battery life than leaving it on standby. If the 'phone allows you to fully discharge the battery before recharging, do so every 10th or 20th time. Et caetera...