Attention: if you get sick reading about needles etc... what are you doing in this topic anyway? But don't read on...
I work at the bloodbank in the Netherlands. So here it goes. By the way: in the Netherlands, no blood or plasma donor is paid. It is a voluntary thing.
In the Netherlands, donating plasma costs 1-1.5 hours, that is including the check-up beforehand (whether you're fine to donate plasma on that day) and the post-donation sit-down-and-have-a-drink. The actual donation process is anywhere between 0.5 and 1 hour, depending on your blood (if there are more cells in there, the process takes a bit longer - don't worry, it does not matter for your health but it does matter for the plasma aferesis process).
Your veins have to be good enough for the process to work, too. Some people's veins are just too thin or too fragile. A needle goes in one arm. First half a liter of blood is taken out, then the machine works its miracle and separates plasma from blood cells. Then the blood cells come back via the same needle. At least, this is how it goes in the Netherlands! This process is repeated 3-4 times until half a liter of plasma sits in the bag and the blood cells have been returned to the donor each time.
Side-effects (rare, but possible) are small hematomas near where the needle got in, dizziness and fainting (always prepare beforehand by eating enough, drinking enough fluids (NOT alcohol), sleeping enough the night before).
Also in the Netherlands, the regulations for men-having-sex-with-men are the same as in the UK, so donating is not possible.
General regulations are that you are healthy (no cold, no influenza, no use of antibiotics in the last two weeks; no heart disease of course, some chronic diseases are allowed but certainly not all, no malignancy in your personal health history either). Also you should not use harddrugs, and your sexual behaviour should be rather safe (not prostituting yourself, no sex with prostitutes, as a woman no sex with a man who had ever sex with other men, no sex with someone who has a sexually transmitted disorder, no diagnosis of an STD yourself in the last year). Finally, there are a lot of countries in this world where we'd rather have you've not been in the last 1 month to 12 months (depending on the country). For example people who visited the US are barred for 1 month because there's West Nile Virus transmission in the US (and not in the Netherlands).
Some rules are there to protect the donor (for example the rule of heart disease), some are there to protect both donor and recipient (current infections), some are there solely to protect the recipient, and we're rather strict with these, because we want blood (and plasma) to be safe for the recipient.
Plasma can be used to distill proteins from. These often go to people who have a hereditary disease that causes them to not have this protein, so the protein from other humans then helps them survive. This often means that it is a life-long treatment, so we need to decrease risk for these people who may get a treatment every week or every month.
Plasma can also be used as it is taken from the body. It often then goes to burn victims who lose a lot of fluids and proteins from the skin where the skin is burned. It also could go to people who are losing a terrible lot of blood because of a trauma. You can assume these people are very weak, so also for them (even though the treatment is much shorter) it is vital that the plasma is safe, contains no infections.
In short: even when in your country you get paid for plasma donations, do, please, understand that being honest about your behaviour and your current health is important, even if that means that you cannot donate plasma (not this time, or not ever).