Good question, but with some very a very complex answers I'd have to say, mostly because of the amount of approaches that could be taken.
From the best memory of my experience (which excluding this round last was 2008)
If by long haul, you mean distance is greater than 3000nm, and with aircraft with more than 50 seats and worth more than 24 mil, definitely wait until you are sure you have found yourself with a sold, strong network within 2500 nm of the base (and that's for the airline to decide, because of where they may be based. Think about flying from an airport like, let's say Budapest, which I believe is typically listed in the game around 8 million passengers. I believe one could get away with it (though with a smaller aircraft, like a IL-18D, though a few IL-62s wouldn't be bad for routes that lead to the Megaports abroad) with at least 50 flights, due to the fact that it isn't an airport with a lot of cut throat competition (like your CDGs and LHRs) and is a fairly decent size. A smaller airport, like, let's take Kaliningrad for example, which enjoys the same geographical advantages of Budapest, but a far smaller passenger size should look at far more.
I think in most, if not all cases, in this game, an airline should focus more on the regional/domestic flying, especially as the maintenance costs of large aircraft suited for long distance travel (like even the IL-62) will really take a hard hit. But if you know smaller aircraft that can do the job, I'd say go for it. Much smaller cost (sometimes like a fifth of it), with a third of capacity and maybe 1000nm less range and 100 kts less cruising speed can be surprisingly worth it. If you're playing a 60s/70s round, something like the IL-18D or Tu-114D is usually very well worth it's money. But as with planning aircraft for your routes, one has to be careful and plan well ahead even when first getting these.
Here is a good example of what I had in Public World #9:
Antonov An-10A 18
Antonov An-24V* 73
Boeing 727-100 Executive 4
Breda-Zappata BZ-308 27
CSS-12 4
Ilyushin Il-14M 45
Ilyushin Il-18D 16
Ilyushin Il-62* 4
Industriewerk Il-14P 6
Nord 260 Super Broussard 1
Tupolev Tu-104A 3
Tupolev Tu-104V 13
Tupolev Tu-114D 37
Tupolev Tu-134* 35
Tupolev Tu-134A* 70
Yakovlev Yak-40* 26
*Launch Customer for specific type.
I had as bases:
Rzeszow
Leningrad
Kiev
Valencia
Milan Bergamo
Budapest
Odessa, Ukraine
So you can see, I was based out of some small to medium sized airports. I recall by the end of the round, I was the 14th most valuable active airline and still rising at that point.
But as I said, things may have changed in the years since I last played. The costs may have been revised, as well as other algorithms (though from what I feel, it doesn't look like so).
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