The answer to this query is: Always (except in times were a site is maybe having bad widespread technical issues, in which case go onto a different site).
If not online you wont have the potential to make any earnings, if online at least there is potential. Other people not doing well on a certain site may mean they have less than ideal equipment/not the right site for their niche or it can be too many models vs not enough clients. In any case we always recommend to have at least 1 back up site. Try to find the site that suits you best, the times that suit you best and the niche that works best for you. If all else fails, content, endless content.
It can be quite surprising how trying out a new site can work for a member that may be struggling on their usual site, the money is out there, you just need to find the right combination for you.
We use to be apart of a popular online community (which no longer exists), this question would appear in various forms and sometimes members would reply "the site is dead"/"no point logging on"/"havent made any money for days on there" etc, yet as we could also see their earnings we would know they were telling fibs, competiton in this industry can be high, if they can persuade other models not to login then there is more clients for them essentially.
Nowadays this can also exist in whatsapp group chats, we would see a member either post about the site not being busy (yet we can see they are busy on the site) or they may post the opposite and say they made £xxxx.xx value on the site (without evidence) to make you feel bad about not making the kind of money they portray themselves to be making and we can also see thats not the kind of money they make. Psychological warfare against possible competitors.
So if asking this query in a public space, always take the answer with a grain of salt and also take this footnote with a grain of salt as most members will be genuinely trying to support others by showing their earnings, to motivate other members of their community to get online. Just something to be aware of.