In a previous life I worked in the cleaning industry and we had access to a booklet called "202 Cleaning Times" published in the USA by the ISSA - International Sanitary Supply Association.
A wonderful source of information, you could accurately judge the time taken for any task from cleaning toilets, to scrubbing floors to window cleaning and get a reliable result.
Is there any such info source in the UK for building maintenance tasks? For example, changing a toilet siphon, fitting skirting, painting a wall?
I bough an RCIS book last year which allegedly quoted costs for various building refurbishment tasks, but at their prices I would either be a millionaire or go skink through lack of business - I suspect the latter.
I'm getting a lot better at estimating two years down the line, but still scratch my head sometimes.