Both USCIS and the Department of Labor publish some statistics on that. It takes a long time.
The PERM certification consists of two bits whose current average processing times are listed here. The prevailing wage determination is first and is currently taking an average of 70 days, while the PERM processing time is taking an average of 156 days if you avoid an audit, so that totals around 7.5 months. Note, however, that this doesn't include the time for 60 day recruitment period, since this could (if you want to live dangerously) be done at the same time as the PWD at the risk of having to do it all over again if the PWD number is unexpectedly high. I also know that these times can be highly variable, so it isn't clear that this is telling you much about your own particular case, but I think people who accomplish this in 7 or 8 months are pretty happy about that and not everyone is happy.
Once PERM is done your employer can file the I-140 petition for you. If you are in the US at the time, want to adjust your status and your visa category is current the I-485 adjustment of status and associated forms can be filed at the same time. Processing times for these can be found here and, for the I-485, can depend on the field office that handles it. If you file them together they may proceed somewhat in parallel (and the I-140 for someone who has already managed PERM is usually uncontroversial), so the I-485 time alone may be the most representative of how long this will take. At the field office closest to my house the I-485 processing time is currently 8.5 to 16 months.
So, all-in-all, you are probably looking at 1.5 to 2 years to get LPR status if all goes reasonably well, plus a month or two more for the card to arrive in the mail. It can easily go longer, though. I have heard that people who want to speed this up will sometimes opt for premium processing of the I-140 (15 days if there are no objections, for $1000 or so extra) followed by consular processing, though stuff can go wrong with that too and if you need to wait out the PERM process anyway a savings of a couple of months isn't going to make a huge difference.