So what's a reasonable quarantine period for new plants?

[ALToronto]

I'd always thought a couple of weeks would be sufficient, but I got a real surprise last night. I had purchased about a dozen new plants (orchids and other epiphytes) for my new living wall. I had these newcomers sitting in their own plastic bags for two weeks before I placed them on the wall, and added about as many plants from my own collection. And they've been happily co-existing for about 3 weeks. Here's a photo, the wall was displayed at the Green Living Show at the end of April:



Last night, I looked very closely at my variegated hoya (lower right quadrant) because I thought it was sprouting a bunch of new growths. Instead, I saw big, fat, fuzzy mealy bugs gathered at the leaf nodes. So that's over 5 weeks from the date of purchase, and I've never had mealies on my plants.

Needless to say, I squished as many as I could find and sprayed the plant and its neighbours with SucraShield very thoroughly. But I'm still puzzled by the amount of time that it took for the infestation to show up. Do I really need to quarantine each new plant for 6 weeks?