Easy answer, you do it when you can afford to do it / raise the finance in a manageable manner.
Also have contingency and budget for 3 to 5 years of job seeking.
Don't really believe you can time it to the economic cycles, by the time you start and finish training you can get complete reversal in the industry.
Pretty sure most of those training who started training in 2007 thought they were training in good times.
Really not sure what "lesson you intend to take in your University Aerospace Engineering department". The majority of an aerospace degree is mechanical or system engineering based subjects with a heavy emphasis in maths.