Very rough estimation would be a bit less than a month if we were still in 2020. However, much of gas consumption has disappeared. Gas is not a significant part of Finnish energy sector. Most of the gas is used in industrial applications with industrial level bakeries being a big consumer. Most of these have moved on to different energy sources after the war. The consumption dropped 50% just in 1st half of 2022. So I would assume that we are at max 25% of the level we were in 2020 which would suggest that this gas lasts multiple months at least.

Edit: I forgot to add the actual point of this ship. This ship isn't really a tanker for LNG. It is a hub for LNG. This ship will remain in the harbor while other ships fill its containers with LNG depending on need. I have seen estimations that it would be filled every 2 weeks since this ship will deliver LNG to Estonia through the pipeline from Finland to Estonia.