Pretty normal in GA ops. I worked a while ago for a fancy(just from the outside) bizz operator with that shiny silver jets and that bit of red inside. Normally 17 on/13 off(off which at least 2 are stby, some holidays, at the end you end up with 7 off/month).
And yes, during their 17 on they EXPECT you to be on call 24/7. I brought that once up during a safety/CRM(lol) course down there in Malta - guess what the "trainer" told me - "if you do not like it, get another job"(thanks for that, I did). So again, usually business jet outfits expect you to be available for 24 hours while on duty(that depends how long their duty "streak" is), but in one of the famous ones it is 24 hours a day for 17 days in a row. In between you fly, have you legal rest afterwards, then it goes on again with permanent stby.
Usually at the most inconvenient time(e.g. you are sitting with your crew at dinner and maybe having a beer after 8pm) the phone rings with a stressed guy from ops and asking you HOW long it would take you to inform your crew, check out, make it to the aircraft and be ready - a clever boy always says it would take at least 3 hours as you are not in the hotel but in the city(whichever), so they need to add the time to make it out of the city to the most of the time somewhere in the middle of nowhere hotel. I guess most know by now about which operator I am talking. But they are not the only ones pulling tricks like this.
Most Business Operators do not have certain stby times (e.g.0200-1400z or 1100-2300z) like ALL airlines do, they plan "duty" for all the time. And the bad ones(which are 95% of them) are giving you your 36 hours AFTER they happened(because nobody booked a flight, they find a bigger then 36 hours window somewhere and put that in your roster). Legal you are asking? Nope, of course not. But do you think someone cares in GA?
Basic advice - IF something in GA smells "fishy" it usually is. Take care.