AFAICT generally people speak the same thing at home they speak in public:
* people who speak Hochdeutsch generally also speak Hochdeutsch in private
* people who speak Swabian or Bavarian generally speak it both at home and in public
* most other regional dialects have moved closer to Hochdeutsch, usually retaining only a few "loanwords" and the accent
A great example of the last one is the Cologne dialect: because of its association with Karneval it is frequently emulated in TV to the point that what most people think of as dialect is just a Rhineland accent with a handful of dialect words thrown in for flavour. The result is comprehensible for speakers of Hochdeutsch but about as authentic as mock "ebonics" sound to an actual speaker of AAVE.
The Cologne dialect of course still exists and there are some people who still speak it but retain a heavy accent when switching to Hochdeutsch. But it's become exceedingly rare even among natives.
Similarly what passes for Hochdeutsch in Swabia is often still heavily accented and full of Swabian phrases to the point that the state of Baden-Württemberg ran a campaign with the slogan "We can do everything but speak Hochdeutsch". "Foreigners" are often confused by the Swabian phrases that are mixed into what Swabian speakers would otherwise consider their best attempt at Hochdeutsch.
Compare this to Saxony "Hochdeutsch" which is often very difficult to understand due to the thick accent but tends to contain far fewer dialect vocabulary.
In other words: when talking to Swabians in "Hochdeutsch" you might generally understand the gist of what they're saying but will have to ask for clarifications on certain words they're using; when talking to Saxons you'll probably just have to ask them to speak more slowly and enunciate words more clearly ("What does a Saxon say when he wants to buy a Christmas tree in New York? 'Attention Please!'").
These are all generalisations but I'm a Cologne native who spends a lot of time in Westphalia and have been to various parts of Germany, so I think my observations might be somewhat representative.
The general historic trend seems to be that dialects have shifted towards Hochdeutsch vocabulary (with a few notable exceptions like Swabian and Bavarian) and in most places even the remaining accent has faded almost entirely.