It's up to each player, but I'd say 2x is probably an absolute minimum on routes of moderate length if they connect major airports. To me, the idea is to accept that the level of profit (and rapid growth) afforded by "normal" play on AM is unrealistic in and of itself, so you try to find the best balance. In the past, when I had an airline based in HND, I flew at least 4x daily to airports like PEK, PVG, ICN, HKG using 2 widebodies and 2 narrowbodies. A route like HND-SIN used all the time on an A310-200 by itself to fly at 1.5 frequency, and even then I still used another 0.5 from a second A310 to get to 2x daily. I flew major intra-Japan domestic routes like HND-KIX at 4x daily on a mix of narrowbodies and ERJs and tried to fly smaller routes at least 3x daily on the ERJ (using slightly smaller birds can help increase frequency without flooding with seats - maybe ORD-DTW is better served with 4-5x E75 than 2x A320). When it came to longhaul, I made virtually every route at least 1x daily, even ones that were long enough to require two birds at 0.5x like HND-IAD. However, I don't think anyone holds reduced frequency on longhaul against you.
Of course, this is all just my opinion - you should be here to have fun and try something different! It's not your problem if I'm sour about people who fly 1-2x on routes out of a HUGE hub like ATL while I'm making virtually zero money flying 3x or 4x daily on tiny intra-Iran routes...that's my problem.
(In reality, I don't care what other people do unless they regularly show up with unrealistic operations on the same routes as me and sap my ability to make money on my few profitable routes.)
While on the subject, I personally would like people to stick to realistic routes as well - don't just throw 2x A319 from ATL to every airport in the USA, including ones listed with as few as zero passengers, simply because you can rake flying from ATL to anywhere. That's probably my biggest pet peave as the ultimate goal here is a realistic network.
I'd say there are also realistic scenarios for 1x daily, by the way. If you have a hub structure of SEA/DFW/ORD/ATL, it probably makes sense to have multiple daily flights from ATL, ORD, and DFW to an airport like MCI. You probably can't really justify multiple dailies on SEA-MCI, but it's pretty realistic to supplement spoke service with a single daily flight to more distant hubs. I've also previously operated 5th Freedom hubs similar to what Delta does at NRT, so I schedule inbound flights to that hub from many cities in my airline's home country, then several of those continue onward to another city in a third country. In this world, though my airline is based in Iran, I will probably have a hub in Dubai so that I can connect several Iranian cities there and then probably tack on a couple continuing flights to places like Muscat, Jeddah, and Kuwait at 1-2x daily.
Anyhow, I've gotten longwinded enough here! These are all just my opinions and I'm sure others disagree with some or all of the above. Regardless, I'd say you should go ahead and play! It's a fun change of pace from the typical worlds.
