It appears a number of factors come into this, such as what point proofreading should be done, familiarity with the text and length of time away from it.
According to Jane Cogie, Kim Strain and Sharon Lorinskas
It is important also that editing [proofreading] be done at the appropriate stage of the writing process, that is, as the last step, after issues of organization, coherence, and flow have been addressed.
In a study, The generation effect in reading and proofreading, the authors Meredyth Daneman and Murray Stainton conducted the following experiments in which;
Subjects spent 30 minutes composing an essay on student life; after a 20 minute interval (Experiment 1) or a two week interval (Experiment 2) they proofread their own essay, another subject's essay after being familiarized on an error-free version of it, or another subject's essay without the benefit of a preview. Experiment 1 showed that subjects were less able to detect errors (e.g., The best part of student like is socializing.) in self-generated essays than in unfamiliar other-generated essays; on the other hand, they were better able to detect errors in familiar other-generated essays that in unfamiliar ones. Experiment 2 showed that the disadvantage for proofreading self-generated text is likely a by-product of extreme familiarity rather than any special quality of self-generated knowledge per se
Commenting on the above study, Maura Pilotti, and Martin Chodorow (doi:10.1007/s11145-007-9110-x) state that;
The difficulty experienced by proofreaders with their own writing occurred even though the proofreaders were warned that errors had been added to their writing prior to proofreading. Interestingly, the impairment uncovered by Daneman and Stainton turned into an advantage after a delay of 2 weeks between writing and proofreading. The researchers argued that the ‘‘excessive’’ familiarity afforded by one’s own text, which is likely to decline over time, is detrimental to proofreading because it promotes a less thorough processing of written material.
It would appear from the above that 20 minutes is too short a period, and that something like two weeks is more optimum, but if possible leave it as long as possible.