Where I work now, we have a regular customer who is like that but with oil and vinegar. We have a single bottle that the oil and vinegar go in, which we shake before squirting it on the sandwich because, you know, the two famously don't like to mix. There's a very fine tip on it of course, and normally a sandwich gets a few passes back and forth for coverage.

It took a little while to find where the line for enough was with the customer, but eventually I worked out that just unscrewing the cap, which leaves about a finger sized opening, and dumping that across the sandwich top and bottom, with a little overrun on the sides, finally stopped the requests for more. I still give him an extra side cup just in case. That dude just loves him a soggy, oily and vinegar-y mess of a sub.