How often does it actually happen?
Also, is it fine to ask them to slow down if it’s hard to understand, for example, due to a heavy accent?
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How often is hard to quantify, but in general I would suggest the answer is all the time.
ATC is there to help you, as a service to you as a pilot. Sometimes out of necessity things are said quickly and it is the nature of the beast that sometimes some information is missed, or unclear. NEVER feel unable to ask for repeated information, or clarification if the information is unclear or ambiguous. Better to take up some extra airwaves than make a mistake!
If the information was delivered too fast and/or some was missed, be specific with what information was missed. This is preferable to asking for all the information to be repeated. A typical scenario might be
ATC: G-ABCD, Runway 22 in use, QNH <unintelligible>, taxi holding point Delta 2 via Apron, Alpha and Delta, giving way to the Cessna now entering the Apron.
G-ABCD: Runway 22, taxi to holding point Delta 2 via Apron, Alpha and Delta giving way to traffic on the apron, say again QNH, G-CD
ATC: G-CD, QNH 1015
G-ABCD: QNH 1015 G-CD
As you can see from the above example, the readback of received information is important, and ask for clarification of the missed items only.
In one study undertaken in France, 3% of transmissions are requests for confirmation, and a third of those are "say again". The reasons are mentioned in the quotation below:
Errors and misunderstandings: these can be directly traced to voice channel characteristics (i.e. its volatile nature, national accents, or poor quality of the audio signal). Such events have a direct impact on the efficiency of R/T use, if not on safety. As an example, one erroneous pilot read back was recorded per hour (mainly wrong frequency during transfer of communications), and led to an explicit correction. On average, the Vocalise samples contain almost 6 requests for repetition or confirmation originating from pilots per hour, and 2 from controllers, approximately accounting for 3% of the total R/T occupancy (up to 14 such pilots requests can even be found in a single one-hour sample). Among these, 30% are simple "say again" requests, and another 30%, requests for repetition or confirmation of route information.
— Graglia, L., B. Favennec, and A. Arnoux. "Vocalise: Assessing the impact of data link technology on the R/T channel." 24th Digital Avionics Systems Conference. Vol. 1. IEEE, 2005.
See Jamiec's answer for how to efficiently request such repeats.
Also is it fine to ask them to slow down if its hard to understand for example due to heavy accent?
The ICAO standard phraseology manual includes the phrase SPEAK SLOWER meaning "Reduce your rate of speech".
Source: ICAO Annex 10 - Aeronautical Telecommunications - Volume II - Communication Procedures including those with PANS status (viewable here)
How often can it happen or how often should it happen are different questions from what you asked. You asked how often it does happen. I’d disagree from the “all the time” answer. In fact, in my experience, considering overall operations, it happens rarely. That’s not real quantifiable, so I’ll say that when I was flying a lot, there were days when I heard it maybe once or twice in a day. There were also days when I heard it more often. It just depends. However, if you’re from the South (U.S.) where we speak fairly slowly and your trip takes you up to the NYC area where they speak a bit more rapidly and with a different accent than in the South, you may find yourself needing to ask the controller to “say again” or “confirm heading two-seven-five after Alpha”, etc. There have already been some good suggestions in other answers, but the bottom line is your main job is flight safety. If you didn’t completely understand controller instructions, it’s not at all safe to continue until you do understand. Do whatever you need to do to get there (i.e., to a point of understanding).