Although Avonelle Lovhaug has an excellent write up, I'll give my two cents as an answer. First off, the only retainers I've used have been with 2 very good clients that I've known for years; they know me well, and I know them well. When I did the retainer agreements, one of them was a lawyer, and I trust him to draw up the contract, and allow areas for us to write in numbers and dates. The reason? It's a legal agreement, and I can change it as needed in certain areas.
What the lawyer did was sit me down, and ask me in plain English what I wanted. I explained what I wanted, and they added some more suggestions that seemed good. For example:
- Date it starts
- Monthly Renewal date (if applicable)
- Notice before cancelling the contract
- My right to stop working if the retainer is used up
- What hours I'm available for regular work (my normal price) and my emergency rate (double my normal price). This also included how long of a response time I was guaranteeing, as well as offering the ability to send in someone else if I'm not there. (Personal Note: I am one of the higher ups for other IT contractors, so I know many of them. I've brought in others for helping with projects before, and most are available at a phone call for me if I'm not available for a project or emergency)
Then, of course, blank fields for the customer's business name and address, contact information, etc. The clients usually have their own agreement(s) to sign as well (i.e. Non Disclosure Agreement), and they get signed at the same time. Now, back on topic...
With a retainer, you need to remember that you are basically becoming "on call" for the client, and usually also need to give priority service. Because of this, I do not feel I would do more than two or possibly three at a time, unless I have staff under me. I don't think the clients would feel good to hear "Sorry, but I got another priority client who paid the retainer this month, so you'll have to wait your turn". Now, back to your 3 questions...
My retainers are unlimited, and sold as blocks of time; usually, 30 hours each time. My clients do not require me to be full time at their office, unless we are doing a project (i.e. refreshing all systems). Because of this, I know I will work between 15 and 30 hours (15 to account for any "emergency" calls). I keep track of it with an invoice sent from my phone billed to the account instead of asking for money. Once it says that the time has been used, new retainer starts up.
Again, depending on your industry, this will vary greatly. For me, when I do good work in IT, I only need a couple hours of maintenance per month per client. I usually reserve the first half of a day doing maintenance/billings. Once that's done, it's just work for the rest of my time. I do not do retainers with an end date, just a length of time they need to give before cancelling the contract when they have money left over "in the account".
Again, because I don't do the traditional retainer you are thinking of, this doesn't apply to me; what I would probably propose though is to have a certain cut off that will roll over to the next month; for example, 75% of whatever is leftover from the previous month gets pushed into the next month, free of charge. Again, I don't do this, but something you may want to consider. My reasoning for not giving ALL the money is because you still need to be paid just for being on call, period. Even when you aren't working, you need to be making money since you are essentially promising to be available.