Typically a lot of papers set their own boundaries for what they want to define as a distal enhancer, especially when the distance is negligibly large, in the case of this paper:
they defined distal enhancers with a range of 80kb - 114kb up or downstream since that is what they observed.
While this article:
defines distal as >1kb and proximal <1kb. It depends on the context of what you're doing but typically it's up to the discretion of the researcher once you've reached a large enough distance of around 1 - 2kb upstream.