It's a YMMV situation. For instance, right now I'm ruing the fact that Mendez/Lebedyenko have done well enough in their few doubles entries (the GSLs plus a couple others for form purposes) that they reached the top 200 and can't play in futures-level doubles tourneys during the year-end dead period.
I would say quit fairly early--once you're playing challengers and maintaining form. After that point, doubles becomes a tool to keep up players' form during the peaks and troughs of the tournament schedule.
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