If I begin upvoting today from my steemit account at 100%, how many people will I be able to upvote before my voting power swooshes down to 0%?

Each full upvote (i.e. upvote at 100% vote weight, or normal upvote if you don't use the slider) multiplies your voting power by a factor of 98%. So technically I don't think voting power ever reduces down to zero but 228 upvotes would reduce your voting power down to below 1%.

A single 100% weight upvote is recovered in 2.4 hours (2 hours 24 mins) on a linear basis meaning that you can make 10 full upvotes a day at 2.4 hour intervals without diminishing your voting power. This is the timing process used by bid-bots.