This is a study that analyzes whether computer science papers include source code that makes it easy to reproduce their results.
The study found that out of 601 papers analyzed, 139 included source code that could be obtained without contacting the authors, and the study's researchers were able to email authors to get the source code for an additional 87 papers.
Of the 226 papers the authors obtained source code for, they were able to configure and run the source code within half an hour on 130 papers, without contacting the authors on an additional 64 papers, and after contacting the authors on a further 23 papers. For 9 papers, the study's researchers could not run the source code at all.
These results don't show how much time researchers spend on making their source code available, but it does show how frequently papers are published with accompanying source code and what quality that source code tends to be.
PLL made an excellent comment. I'd like to add it to my answer in case it disappears later:
Just to summarise: the overall success rate should be seen as 217 out of 402. Of the full sample of 608, 206 were excluded for some reason or another --- e.g. their results weren't based on code in the first place. 402 were left that should have contained code.