How long did this process take?

It didn't take very long; it's surprising. The direct answer to the question is about maybe 3-6 months at the most to make the basic design choices and to get it working. After that I iterated the design and fixed bugs based on user feedback and requests.

I honestly don't remember exactly but there were a number of design things I added at the time. One thing that I thought was important was to have no limits imposed by the shell on the sizes of strings or the sizes of anything else for that matter. So the memory allocation in the implementation that I wrote was quite sophisticated. It allowed you to have strings that were any length while also maintaining a very efficient string processing capability because in those days you couldn't use up lots of instructions copying strings around. It was the implementation of the memory management that took the most time. Bugs in that part of any program are usually the hardest to find. This part of the code was worked on after I got the initial design up and running.

The memory management is an interesting part of the story. To avoid having to check at run time for running out of memory for string construction I used a less well known property of the sbrk system call. If you get a memory fault you can, in Unix, allocate more memory and then resume the program from where it left off. This was an infrequent event but made a significant difference to the performance of the shell. I was assured at the time by Dennis if this was part of the sbrk interface definition. However, everyone who ported Unix to another computer found this out when trying to port the shell itself. Also at that time at Bell Labs, there were other scripting languages that had come into existence in different parts of the lab. These were efforts to solve the same set of problems I already described. The most widely used "new" shell was in the programmer's workbench -- John Mashey wrote that. And so there was quite an investment in these shell scripts in other parts of the lab that would require significant cost to convert to a the new shell.

The hard part was convincing people who had these scripts to convert them. While the shell I wrote had significant features that made scripting easier, the way I convinced the other groups was with a performance bake off. I spent time improving the performance, so that probably took another, I don't know, 6 months or a year to convince other groups at the lab to adopt it. Also, some changes were made to the language to make the conversion of these scripts less painful.