The answer to that is "it depends".

Windows are known to work, but not all windows are made equal. Which direction your window is facing and whether you are in an urban canyon can make significant difference. Some windows also have films or coatings.

If you are working in tricky situations, then the quality of your antenna and the quality of your receiver will make a difference (e.g. multi-constellation support, interference mitigation, multipath rejection etc. etc.).

If you do not have access to a workable window, then your alternative options are:

   - Accept slightly lower accuracy and use LW radio instead (e.g. MSF, DCF etc.)
   - Rent a leased line to your country's national time laboratory. They will deliver you certified, traceable time with no roof access required and microsecond accuracy (backed by an SLA, i.e. accurate or your money back).
   - Run a long cable to the roof (if money is no object you can even get super fancy setups that run exclusively on fibre, no coax in sight)
   - Install an internal GNSS repeater system (still involves roof install, but gives you *much* more flexibility indoors for obvious reasons).