Of course, standing is important for internal training but you should know how to do it correctly. I've seen many people do standing and I've talked to them about it but it seems that many don't really know how to do it. But it is important.

Like in Taiji, you have to know the theory (ming li), you have to know how to move the energy (dong jin), you have to get used to the movement (zhao shu) and then you can approach a deep understanding (jie ji shen ming), step by step. If you practise Taiji or Xingyi, standing and movement should be the same. Both are important.

What's special about standing exercises is that you can concentrate on a single thing more easily than in movement, develop it in standing and then try to take it to the form movement. Examples of such things are developing the sitting position and gathering the qi energy to the dantian.

It depends on how much time you have and also on how you understand form training. In form training, can you do the same thing as in standing: meditation, internal movement, training the internal energy? If so, you can do it all the time. You can do standing meditation in the form and also without the form. It should be the same. These should not be separated: "now I do the form for physical movement, then I will do standing for internal training". That's not right, that's not Taiji and shows that one does not understand internal training yet. This is similar for all the internal martial arts: Taiji, Xingyi, Bagua and also for some other martial arts. They do standing meditation in the form, in movement.