Genghis' post tallies with the self-imposed rules of thumb that I use:
1 unit = 1/2 pint beer = small pub glass of wine (125ml) = pub measure of spirits (25ml).
Add a fudge factor on to the above for strong beers or home measures of spirits (oooh my hand slipped!) Also watch those large pub glasses of wine and the new extra-generous spirit measures that some pubs have.
Assume it's adsorbed instantly then you process alcohol at 1 unit per hour, the driving limit is 4 units (and I've just learned the flying limit is 2 units).
Don't fly or drive until at least 8 hours from your last drink unless you consumed no more than the limit in the first place when a bit of discretion is allowed. Leave proportionally more time if you've drunk more than 8 units.
And remember that if you're drinking wife beater (Stella Artois), 8 units comes very quickly.