I'm assuming they are using mineral oil; I've not filled a watch with mineral oil, but I have worked with it. Mineral oil is not particularly viscous; some gentle tapping is probably enough to get all the bubbles out.

Here's a video of a PC immersed in mineral oil with an aquarium bubbler and you can see the bubbles rise fairly quickly:

https://www.youtube.com/watch?v=tUBvWXH1hLs&t=110s