Help How long does suffocation with a plastic bag and drowning take max until you are dead?

[HopefulSleep]

I am thinking about overdosing on rc downers and then putting a plastic bag with rubber on my head so when I pass out I don't feel the suffocation or do it in bath with water in it and try to position myself so that when I pass out I don't feel the drowning.
If I can't get the money for the downers I am thinking about using sn instead to pass out.
The reason behind this is that I am sure that suffocation/drowning is faster than sn but how fast exactly is it at max?