I put it on a small amount back in early October, and added an extra amount a couple of weeks ago I think. (We have one of those boilers that works with a timer.) In general, my alarm clock has a temperature gauge and I think I want to keep it at at least 17C to avoid accidentally getting mold issues. So it's been varying between about 17 and 20 depending on how recently the heating has been on, and if it gets colder as we go into December I might add a bit more. Initially I had an hour in the morning and 2 hours in the evening, and now it's at 2 hours in the morning, 1 in the middle of the day, and 2 and a half in the evening.
The house we're in has quite high ceilings and this is the first winter we've been in it, so I'm waiting to see how terrible it is in terms of heat loss as we go on. I think in the future we might replace the boiler with something more efficient, since it's quite an old one anyway.