You could also try Dirtbusters Kent for oven cleaning training, I did their 2 day course at the end of Jan and brought their budget tank (gas) for £1295 + vat including training, tank and all the cleaning solutions (non-caustic). Just completed my first month and I'm averaging around 3 - 4 oven cleans a week which is a great add on for my carpet cleaning business.
Time wise it can take anything upto 4 hours for a really minging one including hob and extractor or as little as 1 hour for a pretty clean on. Like most I have a fixed price so I charge the same whether it is only slightly dirty or really crusty!
It is a hard job physically, more so than carpet cleaning and it is not for everyone, it also has a lower price per hour than carpet cleaning but it does help fill odd half days etc.
If you have the money and big enough van I would opt for a van mounted tank from Dirtbusters, costs a little bit more but saves considerable amount of time in setting up etc and you could probably do around 3 to 4 jobs per day rather than 2 - 3 with a budget tank that requires setting up on each job.