When does Numbers sync spreadsheets?

In Numbers, connected to wifi (good bandwidth), after modifying a spreadsheet I go back to "Spreadsheets".

Then I wait for the Arrow ￪ to appear, as it indicates that Numbers is syncing with iCloud.

However, sometimes

• it is instantaneous (does the sync immediately)
• it takes a couple of seconds
• it takes 10~30 seconds

So, on what depends the time it takes to sync? Is it on the clock? Does it depend on the numbers of changes? Temperature? Ground pressure? Pope announcement?

Seriously...