An assessment under the COSHH regs could range FROM - A full blown water system analysis for the purpose of controlling legionella (following L8). TO - A shorter 'sweeter' version of data sheets, user friendly, if there is only a minimal number of products, used in simple ways, and the readership has been taken into account. (Pesticide product labels are a good example of comprehensive but succinct information, these are usually used individually and solely for one specified purpose).
And a host of variations in between. It depends. If in doubt start with TASK, as it is the mixtures, by-products and unintendeds which cause most problems, not the product as analysed by the manufacturer.
I have in the past produced a COSHH assessment for each of about 12 trades (80 workers) in a maintenance unit, using over 300 inventoried products. None of the assessments was longer than 3 pages. Whilst I would now prefer not to combine user information with the management analysis, these were readable for most. This persuaded the HSE to close an investigation and NOT issue a Notice.
Recently I produced a COSHH assessment for a GRP and woodworking workshop. I used one of the example templates on the HSE website (which does actually use joinery as it happens), and built on it (total 4 sides with action plan). To accompany it was a list of the 20 or so products, with by-products, as an index (1 sheet), plus a summary of the key points, for employees, (also used as toolbox talk) 1 sheet. There was also cross-reference to recent hygienist measurements (which validated the risk level and ongoing monitoring).
This was (last month) reviewed by a visiting HSE inspector, who shared their view on how SPs so often just produce replicas of data sheets, and that a real assessment covers tasks and risks. Phew! No further action.
By the way - when I say 'I produced' I mean I did most of the faffing - the research, writing, formatting, organising and compiling. This all went back and forth through workers, supervisors, safety group and management until all were content, and I felt they understood the process and the outcomes.
I must add that over this time an auditor has observed somewhat critically that 'there should be a COSHH assessment for every data sheet' and that he had never seen a COSHH assessment so done (by activity). I felt obliged to point out that this was the template & style suggested by the HSE. There was some tension! But I've had HSE acceptance of this approach, both at the very start of COSHH, and nearly 30 years later, so I feel vindicated. Hope this helps.