I've tried the deep research mode a few times. I noticed a couple of things:
Limited use: Its use is limited, unlike normal chat. But it seems to be something around 25 deep researches per month. I'm on Chat GPT V 5.2 at the moment. Each research takes between 20-50 minutes.
Having the possibility of about 25 deep researches per month is quite a good number (for my purposes). However, there is a small thing to consider to avoid wasting these. If you request a deep research in a new chat, you consume one (rightly so). But if you prompt again after the deep research in the same chat, it tends to go automatically into deep research mode, consuming another of the 25.