Many thanks for your recent enquiry regarding the 635 Litre Compost Tumbler. I've had a read through of your message and have come up with the following answers and information, which I hope will be of help.
COMPOSTING TIME As you're probably aware, it's notoriously difficult to nail down exactly how long the composting process will take, even in a Compost Tumbler. The ingredients and weather are obviously the most important factors but a full tumbler, containing the right mix of green and brown material and including grass cuttings, placed in full sunlight in the middle of summer should produce usable - not great, but usable - compost in roughly 14 days. However, the process - as you say - is likely to take months at most other times of the year, so I certainly wouldn't state that you will get potting mix grade compost in two weeks all year round! The main benefits of the tumblers are the speed of decomposition and the ease of turning and aerating the material, but I'd certainly say you will need some kind of holding area in which to accumulate material whilst the tumbler is at work. If you use a standard compost bin as this holding zone and construct a layered heap with the right materials, it will help quicken decomposition when you eventually add the contents of the heap to the tumbler.
WOODASH It's fine to add ash from a log fire to a compost heap as it provides a good source of minerals to growing plants when eventually applied to the soil. As to the amount to add; it's not an exact science and I'd certainly err on the side of caution rather than chucking in copious amounts of ash, at least to start with. As you probably already know, some compostable material falls into the category of 'green'(grass clippings, vegetable peelings and annual weeds for example) or 'brown' (cardboard, woody prunings, twigs and straw) and the idea is to create a 50:50 mix of both these groups; ash falls into neither of these sections so will not affect the general ratio.
ORANGE SKINS Finally, citrus peel. Adding the peelings of oranges and similar fruits to a compost heap is fine, and there are no restrictions to the amount you can add. These peelings are 'neutral' - both green and brown - so can be added without affecting the balance of the bin, and if you chop the peelings up they will compost much more quickly.