Do you provide your clients with an SLA when it comes to responding to jobs?
I like to pre-plan my day ahead, but I have a couple of clients who send work over and expect it to be done straight away, or ASAP. Which completely ruins my days plans and usually puts me being on a project forcing me to work an evening to make up the time.
I’m thinking of creating a small SLA which states that changes will be made within a set number of hours.
My question is, is this reasonable, i.e. a change to a website is requested, but the SLA states that it’ll be done within 48 hours.
Any advice?
I find that one of the major keys for a web design contract is to not only control expectations as has been mentioned earlier but give your clients a gauge to work from as to what they feel they will get out of the agreement. We put so much effort into trying to sell our services or skills to people we often forget about how we sell our time, energy or sympathy towards the client. As a matter of pre-course rather than having a service level agreement (unless you really want to tightly dictate how you work), I usually recommend people simply produce a statement or section within their general contracts that handles expectations and potential situations (such as in the event of a severe / moderate / minor bug - in respect to costs, time-frames, expectations, support levels, etc). You don’t need to be really heavy or too stringent with your rules, most clients don’t expect immediate results (and you probably don’t want the ones that do) and their usually pretty accepting that you’re not a divine being with limitless resources or time, it just helps them out if you state the after-care support simply and easily. Then they know what to expect before hiring you. 
Seems like a good idea to define and document your service level, sure. It doesn’t sound like an SLA is really needed - maybe just some language in the contract that dictates how responsive you can be.
When we wrote web services contracts, we had a simple paragraph that discussed our level of service. Essentially we agreed to publish formally requested (in writing) changes to their website (content) within 3 business days. We offered urgent service for $225/hr during nights and weekends subject to availability, but clients rarely took advantage of that
I agree that this is primarily about managing expectations.
Providing clarity about how you run your business to the client beforehand is key.
Unless your clients are high level businesses, with really mission critical systems, I wouldn’t have put in place a rigid SLA system, but rather tell the client each time when he can expect to have his changes done. If that’s not good enough for the client, offer him priority service at an additional cost. Be flexible when you can. Offer options when you can’t.
Fire bad clients that can’t be trained. Focus your energy on providing excellent service to the good clients that bring good revenue. You can’t be mr right for everybody.
It is all about managing expectations. Surprisingly, when I tell someone “Thanks, I should be able to have this done for you Thursday”, when Thursday is two days away, they generally do not have any problem with that. It is only when I do not immediately respond to them that they start wondering where their project is and are jumping all over me because their expectations were not met.