If I use HERE's Android SDK and preload map data to the device storage, do I need to reload these data every n days?
We are operating a demand responsive transport system in an area with poor cellular coverage. Having to relocate the cars periodically to some designated wifi access point would be inconvenient and integrating our vehicle dispatcher algorithm with off-the-shelve SatNavs is just not possible.
No, offline data is not expiring, nor needs to be refreshed.
If you are using also online services (and don't operate fully offline) it might happen that some online services (e.g. online routing, traffic, online search) respond with no or bad results with older mapdata (mostly something older than a year). So, if you also have online usecases, please consider a mapupdate every half a year.
Also keep in mind, that outdated mapdata might cause a bad user experience with newly build roads or changed road networks.