There are no one size fits all answer to this. I would say whenever you feel ungrounded. If you are able to dowse or do Kinesiology muscle testing, you can check how grounded you are as a percentage i.e. 0 – 100%.
Personally I find that I need to ground daily and this helps my day flow better if I do this first thing in the morning as part of my morning ritual. The reason is that I often wake up and it’s like I am still dreaming, parts of me are missing. Being grounded means being fully present in there here and the now.
The other thing to consider is what is going on in your life, how stressed you are or if you witness an accident etc. or anything else on the list of things that may makes us ungrounded. Under these circumstances it may be helpful to do a grounding exercise more than once a day.