Do you have any idea how long it takes to syphon all the water out of a bath?

I do now
No, I don't have a blocked drain pipe, rather I had a Eureka moment while relaxing in my bath last night (appropriate place for one, if you think about it

)
My water supply is metered, I pay for every drop I use. I was just about to pull the plug and throw 20 gallons of good money down the drain when I though better of it. I have a young lawn in the back garden that would appreciate a good soaking every bit as much as I did last night.
How long? It took a good half an hour this afternoon to syphon it all onto the lawn.