The first couple of times you can just hit the key a few extra times to show they're doing it, but after a while if they do it all the time this would become tedious to read. Therefore you can just establish it to begin with, then after that make a few references to the fact that they're still doing it, but not actually show it. That's how I'd do it anyway.
I'd just say that the character is trilling or rolling the letters, instead of trying to show it through some kind of strange syntax.
This. There are authors who have rendered accents etc in the dialogue, and there are even some successful authors who have done it. But just telling us the character speaks in a certain way (and then maybe mentioning their funny pronunciation of a certain word later on if it's relevant) is oh so much easier for us to read.
writing dialog phonetically is almost always a mistake... in this case, simply note that the character "spoke with a heavy Scots burr"... or "he rolled his 'r's to a ridiculous degree" or something similar... in other words, describe how the character speaks, as advised above, rather than showing it and annoying the reader with stuff like, "Rrrriicht ye arrre, Rrrrobert, m'lad!"...
