When is the next glaciation due?
This answer assumes 280 PPM CO2, at least until the ice age and oceans begin to draw CO2 from the atmosphere.

If we look at the Pleistocene and we measure how long glacial periods last and how long inter-glacial periods are, You could say, observing the Earth for the last 2.5 million years that being in an ice age is Earth's natural state, with only brief periods between ice ages.  

Ice ages have also followed pretty regular patterns that makes them, in theory, somewhat predictable.   Ice ages mostly followed a 40,000 year cycle, between about 2.5 million years ago to about 1 million years ago, then switched to roughly a 100,000 year cycle over the last million years.  

It's not precisely known why the cycle switched from 40,000 to 100,000 years.  That's known as the 100,000 year problem.  But if we use the current 100,000 year cycle as a ballpark measuring stick or if we measure the length of interglacial periods (which are generally pretty short, 10,000 - 30,000 years), then the next ice age can be estimated at not too far off.  Maybe 10,000 years.

But if we look closer at what actually triggers ice ages, which is (simplified version), colder summers in the Northern Hemisphere, driven by orbital variations.    We can get a better estimate when ice ages should come by looking for when the Northern Hemisphere summers will get colder.

Milankovich cycles don't always neatly line up and the right conditions for the next ice age, based on orbital changes is a bit unclear.  When the red line in this chart below is low ice ages are more likely to happen, and we're in a dip now, but coming out of a warmer summer period and relativly soon entering another one.   There's a slightly bigger dip that peaks in about 60,000 years from now, another one 100,000 years from now and a bigger one, about 130,000 years from now.



Using this chart as a guideline, the next glaciation is more tricky to predict.   Perhaps in 60,000 years.   These predictions are hard to make though because ice ages are self sustaining.

So, short answer, perhaps the best answer.   Nobody knows.   The Milankovich cycles aren't lining up in the right way to make this prediction easy.  We may be set for an unusually long inter-glacial period.

As for how catastrophic would it be.   Well, for one, we'd see it coming and stopping an ice age would probably be easier than stopping ice from melting.  Releasing CO2 and CH4 into the air for example.  For a longer answer, how catastrophic would it be is a world-building question.