Ah, well, these ones are slightly different, you get 'invited' to complete a Ciao survey according to the profile you have completed (interests, age, job, whether you have a car or not, etc etc). Normally you answer a few questions at the start of the survey if they have very strict criteria in terms of who they wish the respondents to be, and if you don't fit them, you don't get to fill in the rest of the questionnaire and don't get the cash.
They're a bit like buses, I'll get nothing for a few days, then 2 come along at once. It maybe averages just under one a day for the two of them; I reckon over 2 years I've made a couple of hundred quid. However, with Ciao if you're able to tick more boxes, eg if you have a car / fly a lot / make purchasing decisions at work (especially IT), all of which I don't, you do get sent surveys more often.
Time wise...some of them just take a couple of minutes, most around 10 minutes; but I've had the odd corker that lasted the best part of an hour.
YouGov surveys seem to be open to all; I've done it for about 8 months and so far made just under £20; they generally pay 50p per survey and their surveys mostly last around 10 minutes.
So, they're not really a sideline for me, just occasional pin money. Also, with Ciao and YouGov you have to complete the surveys fairly quickly; the former close the survey when they have enough respondents; this is often within hours. The latter always have a deadline which is usually a couple of days or so hence.
If your OH wants to check these out, see
http://www.ciao.co.uk/ and
http://www.yougov.com/
HTH.