the first and most important issue is her refusal to engage with colleagues until the Ritual of the Breakfast has been completed. that one’s easy. you just need to ask for a meeting, and advise her that this isn’t working out for the team, and that she needs to address this issue as a priority. you’re grownups - this is not a difficult conversation.
as for the time taken? well, that more depends on where you work, and what the accepted business practice is within the company. is it 9-5 with an hour for lunch? any paid breaks? shift work? out of hours cover/overtime? flexi-time? core hours?
i second what @Teddybear45 said upthread - OP, spend a bit of time observing the rest of your team. who has fag breaks, or always makes the tea, or always ‘breaks’ some tech so it takes an hour to use the photocopier. dental appointments, GP appointments, childcare nightmares, early starters and early leavers. who spends an hour every afternoon researching their next holiday? anyone got a side thing with ebay? who always asks for more to do, and who is happy to stretch tasks?
all this information, taken alongside your business needs and the contractual obligations of your team members, is a good starting point when considering whether the team as a whole needs a review, or whether Breakfast Brenda dropping the Do Not Disturb nonsense is sufficient.
this isn’t news to you, but no member of your team should be irreplaceable. who else has access to all the training materials for new starters? are there any tasks particular to Breakfast Brenda (that maybe are closely guarded and Mysterious) that could form part of a redistribution of work to ensure the competence of your whole team?
if, as you say, the issue of not being snarly at others when Breakfasting may be enough to tip the hand of someone who may not financially need to be there, you should really be looking at a contingency plan.
how involved is your manager in your day-to-day? if you are genuinely not sure how to address this, you should probably talk to them. better that you raise it, than a disgruntled member of your team.
ultimately, life would be much, much easier if there weren’t so many fucking people in it. godspeed, OP. i don’t miss managing people one little bit.