Of course it will depend who you ask and will vary between individuals, but British recommendations are two to three units of alcohol a day for women and three to four units for men - with the proviso that you have two or three alcohol-free days each week. So that equates to around say 15 units for women and 20 for men.
Given that a Stella for example could have as much as 2.5 units per half pint and a can of beer is about 1.5, and a large glass of wine is 2, it doesn't leave much does it.
I estimate that at the moment I am consuming around 15 units per week on the basis that I have cut down in sympathy for my preganant wife.
To calculate the number of units take the strength of the drink (% ABV) and amount of liquid in millimetres (one pint is 568ml; a small glass of wine 125ml). Multiply the amount of drink in millilitres by the percentage ABV, and then divide by 1,000. To make matters easier many manufacturers are now stating how many units of alcohol each can or bottle contains.
e.g.
1 pint of Ruddles County @ 4.7% x 568ml / 1,000 = 2.66 units
1 small glass of wine @ 12.5% x 125ml / 1,000 = 1.56 units