How high can I fly a model aircraft under 14 CFR 101 rules?
There's no specific altitude restriction for model aircraft anywhere in part 101 (that I can see). Even within 5 miles of an airport you only have to inform the tower, per 101.41(e):


  When flown within 5 miles of an airport, the operator of the aircraft
  provides the airport operator and the airport air traffic control
  tower (when an air traffic facility is located at the airport) with
  prior notice of the operation.


However, practically speaking there is an indirect altitude restriction in 101.1:


  For purposes of this part, a model aircraft is an unmanned aircraft
  that is:
  
  [...]
  
  (ii) Flown within visual line of sight of the person operating the
  aircraft


In other words, you have to be able to see the model aircraft and that limits the altitude and distance you can fly it at. It's also worth noting that 101.43 is very broadly written:


  §101.43   Endangering the safety of the National Airspace System.
  
  No person may operate model aircraft so as to endanger the safety of
  the national airspace system.


Arguably, that also implies you shouldn't fly any higher than you really need to.