I don't measure by day. I've been writing continually for years and expect to keep doing that for many more, so I absolutely cannot afford to risk burnout. Measuring by the day increases stress and guilt on non-writing days and leads to paralysis. I measure my word goals by the month -- minimum 15k per month, so an average of minimum 500-ish/day (depending on the month). I reached this number by figuring out how much I can write every day before writing starts to feel like 'work', which is about 1k words. (I can write 2k comfortably, and push as high as 10k for special events like nanowrimo, but above 1k/day starts to feel like a job.) Then I halved that amount, so the word goal would give me absolutely no stress whatsoever (because it's so achievable), allows for emergencies, busy schedules or writer's block, and means I usually finish halfway through the month and have half a month off writing. (When I reach my monthly goal, I have to stop at the next natural stopping point, usually the end of the chapter. I'm not allowed to write again until next month, even if I feel really inspired. This gives time and space for editing, creativity, and planning/getting excited about future plot points with no writing pressure. This is essential to avoid burnout long-term.)