Depends on a trillion factors to be honest. Some key variables:
- how technical/ complex the information you're looking for is
- how much information the firm has posted about them online (e.g. an MC firm will probably have a lot more info than a semi-boutique American outfit or a regional firm
- what you need the research for
- how good you are at researching
- your ability to synthesise the mass of information you'll be presented with
- whether the stars are aligned and the local cats are in a good mood
(I'm being flippant here but you get the picture)
Speaking from experience, I rarely spent more than around 90 minutes to 2 hours researching a firm and I'd consider myself moderately proficient at general research and quite quick at synthesising/ extrapolating key info. I also looked for very specific pieces of information and, by and large, knew exactly where I would find them before I started searching which saved me a lot of time.