At what point can I start to visualise things while meditating?

Wherever you put your attention to, there your energy flows. Visualization happens in the mind. When you direct your attention to the part of the mind that visualization happens, and when you connect with your intention within, you can visualize the forms that you intend. This is your natural ability.

First I advise you to learn to connect within yourself so that you can connect with your intention. Also, a meditative practice will teach you to stay as awareness while your mind expresses its understanding. During this, you will also witness the part of the mind where imagination happens.

Consciously connecting with your intention and imagining can quickly bring in your awareness in your mind whatever you are intenting to imagine. So, train your concentration ability with meditation and discover who you truly are more and more.

The development of the skill will happen as quickly as the strength of your concentration and how deep you are focusing towards within yourself and how fully you are staying as the awareness with everything that happens in your experience.