So, you breathe normally before your breathholds? I mean: just like you would be breathing (without thinking about "how" to breathe) when, let's say, watching tv, or doing the dishes, or... ? Let's consider another way of breathing.
You are already laying on a bed, relaxing, which is very good to start with.
Ok, so, since you are doing this in a safe environment (read: laying on your bed, and NOT in the water!), I'd consider doing do a breathe up:
-) With each breath, your exhale time should be twice the inhale time: if you breathe in for 3 seconds, breathe out for 6 seconds. Or 4 s (in), 8 s (out). Or 5s - 10s, you get the point. Just find your pace. (I prefer the 4-8 myself 3-6 is a bit fast for me)
-) Make your inhales deep by using belly-breathing: make your belly expand FIRST, and only then your chest. (That's a "two-section" breathing) - If you're not used to do this, it takes a bit of trying/training. But really try to do it like this!
-) Make your exhales not too brutal, let the air flow away naturally and don't try to overdo it in blowing/pushing the air out. (Except maybe for your very last exhale just before your last inhale for your breathhold)
Do this for about 2-3 minutes. Then, exhale ALL you can and inhale ALL you can (belly expands first, THEN chest!!!). This last inhale can be quite long (10 seconds or even more...?). And hold it... Relax. And keep relaxing!
Think of everything BUT holding your breath. Sing a song in your head. Listen to music, think pleasant memories. Think of beauty, ... really ANYTHING but holing your breath. Don't look at your watch, not even when contractions start. Try to relax your muscles between contractions.
After your breathhold: let your breath coming to a normal pace again (as you would breathe normally) - this may take some time, and so: take the time indeed: rest for 2 minutes. Then repeat the whole proces: breathe-up of 2-3 minutes... Do your breathhold... And again: 2 minutes rest, then another breathe up... breathhold... Repeating this will probably make your breathhold times increase.
AND VERY IMPORTANT: don't overdo yourself. If you are training alone: do NOT do it in the water. Just don't. Keep it dry. And make sure you can't fall - lay on the bed or something. You have to keep it safe!!!
I could also advise you to do static tables (O2 - CO2), but that was not your question, am I right? Your question was to test your max. breathhold. (Which you need to know for doing tables in the frist place)...
Hope someone with real knowledge about this (yes, I haven't even taken a course myself) chimes in to correct me where I'm wrong. I'm quite a beginner myself.