SLS can fly as often as shuttle--and once flying, should be able to fly with less problems because there is no orbiter.
No, it really can't fly as often because it costs about $2 billion a flight, according to NASA's Dan Dumbacher, who is developing the SLS. It's about a billion for the Orion and a billion for the rocket, and the Orion is probably not reusable either. That's the low
cost estimate - from the NASA guy who's trying to sell the SLS program.
Of NASA's total budget of about $18 billion, only about $8 billion is available for human spaceflight, and $3 billion of that is tied up in the ISS and $2 billion is tied up in support or other programs, leaving about $3 billion for flying missions.
We averaged 4.5 Shuttle missions a year, and if you discount the two downtimes, 5.5 missions a year, with a peak of nine. So just matching the Shuttle's average flight rate would take $9 to $11 billion a year, and to match the Shuttle's peak rate would take $18 billion a year - in a budget that allows about $3 billion. Given that budget, they're not going to be able to afford to launch one very often.
Other issues are that we only built about 40 SSME's over the 30-year span of the Shuttle program, and each SLS launch throws away four. Yes, NASA picked the world's most expensive and complicated engine with the lowest historical production rate of any non-canceled motor to power this monster.
The problem is that NASA needs a larger budget
No, the problem is that NASA needs a cheaper rocket. Each Orion carries a crew of two to six, which is less than the Shuttle. They're not going to get an extra $12 billion a year to fly possibly six (high end cost estimates) to thirty-six (low ball cost estimate) when the same amount of money could buy a brand new Ford Class nuclear powered aircraft carrier for a Navy that's seeing its hull count plummet. That's not going to get past the same Senate that was complaining about buying seats on a Soyuz at $70 million a pop, especially when one of them pulls out a calculator and sees $300 million a seat best case and about $7 billion a seat worst case. Two seats on a glorified Apollo space capsule are not worth as much to this country as a nuclear powered super carrier.
Even if you advocate for heavy lift, it needs to be cheaper, and it shouldn't be wasting precious payload and stack height on a big crew module, service module, and giant abort tower for every mission. You should use a small capsule for going up and coming down, and if you need a big capsule for going far away, don't stick all the extra volume inside the re-entry capsule, do what the Russians do and make an orbital module.