Tom, I note this is covered directly in the section in the lesson explaining that ‘le temps que ça prend…’ uses the indicative rather than the subjunctive so I’m assuming something else is triggering your thoughts about using the subjunctive here ?  

Regardless of the reasoning, when using ‘le temps que..’ to refer to one event by/within the time period of another, the subjunctive is used.

However the use of ‘le temps que ça prend’ is different, in that it is discussing something that is definite as a stand-alone fact - the time it takes -  and there is no doubt, assumption, suggestion or hyperbolic comparison here that something else could happen “by/within” the same time period.

This is discussed in the section ‘Attention’, although perhaps not adequately explained. 

   When expressing for example ‘the time (that) it takes ..’ or what (it is that) the weather is like …’, the indicative is used

I wonder if like me your first thought was that ‘je ne comprends pas …’ might trigger the subjunctive here ? 

However this is not ‘ je ne comprends pas que qqc….’ that would require the subjunctive in a following related clause, but  ‘je ne comprends pas qqc que ..  ‘ and this latter structure does not bring in the subjunctive.