What does Lichess consider to be the rating period for the calculation of Glicko-2 ratings?

Lichess seems to use the Glicko-2 System: http://www.glicko.net/glicko/glicko2.pdf
Since the ratings are updated with each game, I can only imagine that a single game is treated as a rating period.

Yes. AFAIK the variable "rating period" is taken into account when calculating the volatility and updating rating deviation. Therefore people who paused for a long time or play only rarely still have their rating deviation grow.

It's not perfect solution but correct calculation with periodic batch updates would be IMHO not acceptable for most of the online chess community.