Still going. After about a week (187 hours) the 34465A is still happily sampling.
I had interpreted the spec to say the base memory was sufficient to do 50k samples but after revisiting the spec it says 50k readings. I would have thought that each reading occurs when the trigger fires (and that a reading was synonymous with a sample), but apparently the reading is just that (a reading) and a reading involves multiple samples (presumably tied to the PLC setting) so the sample readout is just that, the number of samples - not the number of readings. If I was Keysight I might have chosen to display the number of readings (in addition to also showing the number of samples). In any event at 100 PLC and 100k samples this thing is still going strong a week later and I don't envision testing anything for more than a week.
Looking at the user manual, it really looks like they are using the words "reading" and "sample" almost interchangeably, and "measurement" occasionally as a synonym, too (in addition to "measurement" as the general concept of measuring). It looks like they just call it a "sample" in the context of the "digitizer" function, but that it refers to exactly the same thing as a regular reading. From what I glean from the manual, "regular" readings are taken by auto triggering (where an algorithm decides "oh, I think this is an actual reading and not noise") or from being manually triggered. The digitizer mode, in contrast, takes readings at regular intervals, meaning it's acting as a DAC, so they go with "sample" there.
But unlike with the terminology, the manual is very clear on how the memory works: it's a FIFO buffer, so it simply discards the oldest readings/samples/measurements once memory is full. So yeah, it's telling you it's taken 100K+ readings/samples/measurements, but only the newest 50K are actually being evaluated. You can likely let it go and take millions of readings/samples/measurements, but it doesn't mean the memory is actually holding them all.
There's an SCPI command to query for overflow once this happens.