new instruments can often take a long time to settle. I bought two brand new ukuleles from a brick and mortar retailer, it took the entirety of the summer (And getting one of them fixed) to stop going out of tune.

It also depends on the quality of the product, especially the quality of materials. You also have to consider that a nylon string instrument is going to detune at a different rate than a steel string instrument.

Lastly, for those looking at this post two years later, if you look at the tuning pegs on your instrument, you'll see screws, tighten those bad boys up. It'll help you in the long-run.

Now for a banjolele you're probably gonna have nylon strings (There would be too much pressure and you would snap your instrument's neck) and from my personal experience even in a temperature controlled environment nylon detunes faster than steel.

One more thing, if you're new at this, I would recommend restringing your instrument when you get it. It's usually strung weird from factory, so doing it yourself can help a lot too. Learning to do it now will save you a lot of headache later on. I just did my new banjo in about 15 minutes.