Two hundred fifty words I understand is on the generous side. Screening readers who share their process vary widely on their drop-dead limits. The range of first reading sampling I've seen is from ten words to five hundred with one hundred thirty the mean. One hundred thirty is a first page in Standard Manuscript Format. Partial page sink, title, byline, and thirteen lines text, ten words per line, idealized words. Of course, quite a few submissions don't make it to the first word. They broadcast serious issues before the manuscript is even looked at.
I usually have a clue of the writing caliber within ten words, though I'll read as far as it takes to find something favorable to comment on. I want to read for entertainment, but if I'm put off I'll switch hats midsentence.
Things that change my approach from reading for entertainment to reading for commentary include;
Nondiscretionary mechanical style issues: grammar, spelling, punctuation, formatting
Disrupting willing suspension of disbelief
Ordinary, everyday settings with little influence on plot
Spoiling the participation mystique
Illogical causation: cause ought best precede effect
Flat affect, flat attitude, emotionally indifferent scenario
Flat empathy quotient of tension
No particular curiosity engagement, suspense quotient of tension
Unsettled narrative point of view
Haphazard closing and opening narrative distance
Uncertain reader surrogate and/or attitude holder
Static dramatic action: standing, looking, waiting around, etc.
If I'm not put off, I'll read further, scrutinizing for big picture facets, continuity of concept, a clear creative vision, and a solid plot arc, for instance.
Spread the love of written word.