Anywhere from the 20s to 60s depending on where in the world you were.
The history of pro wrestling started in carnivals. This is how works started and how you would have shooters.
A travelling circus would come to town, there would be a wrestling exhibition on which would feature the travelling champ who would fight for money. In the audience gathered there would be a plant who would get in and 'beat' the champ and 'take' his prize money.
This clearly appealed to the local hardnuts who fancied a go at the 'easy money', but what was always the case was that the man inside the ring was HIGHLY skilled shooter who would tear a limb off the locals in seconds. Once the locals caught on to the scam they were gone out of town off to do it again.
This is where the term 'marks' comes from.
Things became a work when promoters and shooters realised there was money to be made by pulling matchs. Two famed shooters would go up against each other in a highly anticipated match up that would have a HUGE gate, both grapplers would save face by having a closely contested 'match'(work) and make huge amounts of money(for the time).
After the Hackenshmidt/Gotch screwjob most stuff after it was a work.