First I have to admit, I haven't really builld bots with php, just some lowlevel scrapers, but some things I learned with multiple projects / apps I'd like to share.
Also I'm not sure about how experienced you're with coding, so I might tell you nothing new but here we go.
As a starting point have a look at some git repos on github for bot examples
here
With a bot for mid to larger scale you like to have a look into pthreads
here, as you would like to run your scripts in multiple instances / threads. Quick and dirty tut
here.
So to have a good performance you would need to seperate your frontend to a regular webserver enviroment and your actual botting scripts as cli scripts.
pthreads afaik is only available for cli at the moment. You probably want to implement some kind of spooler / queue for those scripts
I'm not sure if you would use the official API of instagram, that may depend on your bot and what your planing to do.
So afaik there might be some problems to login on instagram with curl or something from a php script.
Propably you would need to make use of some headless browser. There are multiple posts to that on sites like stackoverflow, I link you a few
here, and
here.
Depending on the enviroment and settings you gonna run your bot, resources could become a bottle neck. So you could easily run into memory limits.
There are many docs on how to prevent that, to get you started read
here,
here,
here.
How long would it take? Depends on your coding experience, planing and feature set. The basics like frontend, queue, basic bot scripts and stuff like could be easily done in a week or two.
I think the tricky part comes when you start building the actions the bot should perform on instagram. As I'm not into instagramm I got no clue what features you want or need.
Hope I could help you out a bit.