constantly, all the time [Dutch phrase of the week]

Literally translated, a "lopende band" would be something like a "running belt". Actually, "lopende band" is the translation for "conveyor belt" (as part of an assembly line in a factory). The expression "aan de lopende band" is used to refer to the continuous or repetitive nature of a certain event or activity.
In the case of a continuous activity "aan de lopende band" is synonymous to "de hele tijd", which is the literal translation of "all the time", see the Examples.

Examples:
– "Tijdens de vergadering moest Frank aan de lopende band hoesten."
("During the meeting, Frank had to cough constantly.")

– "Het kan toch niet zo zijn dat jij me aan de lopende band beledigt, en dat ik mijn mond moet houden!"
("It can’t possibly be the case that you insult me all the time, and that I have to keep my mouth shut!")

– "Schat, je zit aan de lopende band in je neus te peuteren." – "Moet jij nodig zeggen!"
("Honey, you’re picking your nose all the time." – "Look who’s talking!" Note that the typical
Dutch construction for continuous activities is used here: [subject]  + [conjugation
of verbs suchs as "lopen" / "zitten" / "staan" / "hangen"]  + ["te"] +
[infinitive].)

– "Die man met dat blotebillengezicht zit de hele tijd naar me te gluren…wat een engerd!"
("That man with the pudding-face is peeping at me all the time…what a creep!" In Dutch, "scary" or "spooky" is translated with "eng".)

Related words:
– "Lopen": to walk, to run.
– "Band": band, belt, strip, tire.