Where I work it seems to be constant, in theory it's all voluntary, but we get a lot of reminders and expressions of disappointment if insufficient is collected.
It's things like:
Charity dress down days
Birthdays - big ones for the entire staff, every year ones for the immediate team
Flowers for anyone off sick for more than a week or two
Flowers for anyone bereaved
Charity donations in memory of deceased parents
Leaving gifts
Donations in lieu of Christmas cards
Secret Santa
Other fundraising events
The Charity ones, the boss will set up a just giving page and make one donation on behalf of the organisation, so she knows what's been given by whom and will send reminders as she sees necessary.
Same with birthdays, I'm on the leadership team and she will buy a present for each member and tell us what we owe (which is better than me having to buy the present, I accept 
)
Usually these things are organised by a colleague and easy enough to ignore but this is all coming from the top with significant pressure.
The most recent one is a donation in memory of a colleague's father. She's a good colleague, a friend, I'd never met her father but it's a good charity I don't have any problem supporting but I've had 4 reminders in 4 days!
I think I'm fairly generous with charity and as a senior member of staff I can afford it, not everyone can. It's the demands I object to and I also feel for those for whom this presents a real problem.
OTOH there's no way to raise this without looking miserable and tight.