Generally a trial period of 90 days is only intended for 90 days.
There are ways to work around this, but I cant share them here as for it defeats the EULA of the Trial to make a Trial version as an extended use.
For offline use, if it doesn't require an internet connection to function, it can be loaded as many times as you want until it expires, however any trials that require online registration in order to get the 90 days, microsofts servers will either allow consecutive activations to give you additional 90 day trials or it will state that you need to buy the OS.