I was gonna say 3 days, past 19 slots open. I keep 1 or 2 spare gates in my hubs (where available) to protect against some newbie buying it all up. I learned this from someone doing this gate-buy scheme against me (likely unintentionally) at MSP.
The gate-cap theorem has a few issues in the present game. 1. I legitimately use 160 gates out of 160 leased at MSP, because i don't buy into this unrealistic 1 frequency per connection = über best way to do things (even though it seems to be the absolute most profitable method). On a similar note, I use every single slot of my RSW gates (128 in total), and am frustrated at the unavailability of further gates to fill in at least 3 short-routes that blanked out on me in creation. Similar situations are found in SAT (136 out of 138 gates used) and SMF (95/95 gates).
Its bad enough i have no ability to expand in RSW without purchasing gates, but to cap me much lower without that again is brutal.
The second major strike against it is the game cannot handle competition. From what my preliminary research (basic experience) says: the most profitable is always 100% LF 0 competition on the route. 1 additional airline on a route (2 total) in any frequency combination seems to throw that route down 99.9% of the time to 0% LF on the higher priced route. Significant cuts on price seem to swing routes into the steep negative revenue (I'm talking about a Dash-8-300. 0 hourly. NO stephen, there are no staff/fuel costs hidden to 'take into effect'. I can prove this).
When there are a total of 3 airlines on a route, it can throw a plane somehow into steep negative (100% LF/Negative Income) passengers.
The game is great, it's an awesome framework, but it needs some tweaks (not a total complete and utterly insane rebuild from scratch throwing waaaayyyyy tooo much on the top of it as described in full in the AMO section)