Not knowing when it's going to strike and how long it's going to last is tough. But unfortunately I agree with zinco.. any company just wants to get along..

In school and at work I go with the strategy to say things like "Usually it lasts for a week but I can probably get back and work half efficient after 3 days, we'll see". Maybe it's worse than that for you (probably it is) but I just want you to get the point. Usually it is better to say that you will be away for 2 weeks and get back after those than to say no specific time and be away for only 1 week. The human psyche always seem to need a goal, or something to cling to... kinda. In worst case... you could promise to TRY to get back, or do something regarding your work in some time.

But then I don't know what you work with, what they already think about you, what you are like or how things work in your country. Hope you just don't overlook these tips..