1)There is no published time that am aware of and also can depend if completed on paper or online. Could be 2-4 weeks or more as not probably a high priority for IRCC
2)Once PR renounced then ETA application would fall into same timeline as anyone else, within 72 hours or less I believe
A few people on here have applied for an ETA not realising they were still PRs from years ago, received a rejection letter for that reason .This offers them a couple options , request a PRTD or renounce PR.
In taking the latter they create a MYCIC account, link the rejection letter reference to the account which then gives them the capability to upload the renounce form with a photo and copy passport bio page. Having done that then in a couple of weeks they get an approved ETA, assuminhg of course no other reason not to get a positive outcome.