The glass piece you have purchased has been cleaned, then completely protected with a waxy sealer that is applied in its liquid form, left to dry, and is then buffed and polished with a soft cloth.
Other than a gentle dusting, your piece may never need to be cleaned. If something gets spilled on it or dirties it in some way, clean the piece as gently with a soft, damp cloth. I would not suggest using window cleaners or other harsh chemical cleaners on any piece of stained glass, as they may damage the solder or glass and the strength of the piece. Always use the gentlest way of cleaning first and see if that is sufficient.
If you notice that your glass piece has become dull I suggest reapplying a sealer to make sure that your glass piece is once again protected. Glass sealer/polish is available at stained glass supply stores but is pricey. I recommend using a good quality furniture or car wax which contains carnauba wax. It does perform well, just follow the manufacturers’ directions.