Is there a recording time limit?

No. Webshot records until you stop it. An optional auto-stop ends a recording after 1 to 60 minutes, and a storage check refuses to start when the disk is full.

There is no cap on length and no watermark added afterwards.

Auto-stop

Under More on the Record tab, or in Preferences, switch Auto-stop on and pick 1, 5, 10, 30 or 60 minutes. It is off by default. Use it for a demo you want to keep short, or so a forgotten recording does not run all afternoon.

What limits length in practice

The recording is written into the browser's storage as it runs, so the real limit is free disk space. If there is not enough room to begin, Webshot says "Not enough storage to start recording". Clear browser storage or free disk space, then try again. Long recordings at 4K and 60 fps grow fastest; 720p at 30 fps, the default, is the most economical.