In OBS, add a Browser Source, paste this address, and set the size to match your scene. Leave "Shutdown source when not visible" unticked: that option unloads the page, so the countdown would start over every time you cut away from the scene.
A stream needs a clock the viewers can see: how long the break lasts, how long until the subathon target, how many minutes are left in the segment. This builds that clock as a web address, and OBS treats a web address as a source like any other. There is nothing to install and nothing extra running on your machine.
Set the duration, label and size above, copy the address, then in OBS add a Browser Source and paste it in. Give the source your canvas size, usually 1920 by 1080, and leave the background alone: the page paints none of its own, so only the digits land on the scene. The preview is that same page at canvas scale, which is the only honest way to judge whether S or XL is the size you want.
One setting is worth getting right before you go live. The address carries a duration, so the countdown starts over whenever OBS reloads the source, which is exactly what a break timer dropped into a scene should do. What you probably do not want is "Shutdown source when not visible" ticked: that unloads the page every time you cut away, so the timer begins again each time you come back. Leave it off and the source keeps its place, because the countdown is anchored to the clock and works out the right number the moment the scene returns.
At zero it does one of three things and you pick which: hold on 00:00, disappear, or start counting up, which is the one for seeing how far past the break you have run. Leave the outline on. A two pixel halo is what keeps the digits readable over bright gameplay.
Anything that can add a browser source works, which covers Streamlabs, OBS Studio and the streaming software built into most capture cards. The overlay is an ordinary web page, so nothing about it is specific to one program.
Yes. OBS suspends a source whose scene is not showing, and the countdown is anchored to the wall clock rather than counted in ticks, so it works out the right number the moment the scene comes back rather than resuming where it was frozen.
Pick one of the five themes in the builder and it is written into the address. The digits are drawn as pixel blocks rather than text, so they stay sharp at whatever size you scale the source to in OBS.