It happens when exactly 87,855,768 of the possible combinations have been played, but what that is in terms of actual sales is difficult to say. It doesn't even come close to happening with sales of "only" 100 to 110 million tickets as can be expected for the current drawing. Selling 50% of the possible combinations would require selling about 160 million tickets if the combinations were all picked randomly. Since a lot of people don't pick their combinations randomly there are even more repeats than would result from simple probability. Here's a simplified explanation.
Let's imagine that the first 175,712 tickets had no repeated combinations. That would cover 1% of the possible combinations. With 1% of the combinations already played we would expect that 1 of the next 100 tickets would be a repeated combination, so selling 100 more tickets only increases the number of combinations that have been played by 99. Selling another 175,712 tickets would add about 173,955 new combinations and 1757 would be repeats. After 351,423 combinations have been played 2% of the combinations will have been played so only 98 of the next 100 tickets will be new combinations. After527,136 (3%) combinations have been played 3% of subsequent tickets would be repeats, and so on.
To simplify things we can figure that for the first 1% of combinations every ticket would be a new combination and we'd have 100% efficiency. For the next 1% we'd only have 99% efficiency, 98% for the 3rd 1%, and so on. 100% is 1.00, 99% is .99, 98% is .98, and so on. That means that for successive batches of 175,712 tickets sold we'd get (175,712 X 1.00) + (175,712 X .99) + (175,712 X .98) + (175,712 x .97) etc. new combinations. That's the same as 175,712 X (1.00 + .99 + .98 + .97 ...)
If 87,855,768 tickets are sold we'd get 175,712 x (1.00 + .99 + .98 + .97 ... + .53 + .52 + .51) which comes to 175,712 X 37.75. That means we'd only expect 37.75% of the combinations to have been played instead of 50%. That's a simplification and 37.75% isn't quite right, but nobody wants me to start using calculus.