Once I reach a nimitta during jhana meditation, do I then move focus to it or keep in the object? Do I ignore the nimitta, or is it one of those things that is very hard to ignore?
I presume that your object is the breath, crossing the anapana spot (under the nostrils, on the upper lip).
You must be careful since wanting the Nimitta to appear will actually prevent it from appearing. As long as you desire such things, to experience Nimitta or Jhana, it will simply never happen.
Let go of such expectations, completely. Also, by wanting Nimitta to appear, there is a very high chance of it being the product of your imagination.
You must let it be, do not give it any attention. Nimitta will strengthen, stabilize, and merge with the object on its own. Always remain with the object.
Nimitta cannot be ignored in jhana because the nimitta itself is one-pointedness (ekaggatā), where the mind (citta) itself becomes the nimitta & the mind is glued unmoving to the nimitta.
An analogy is a wheel that is fixed to an axle and spins around the axle. Being fixed to the axle is the nimitta/ekaggatā. The mind being expansively conscious, luminous & blissful is the wheel.
In the Pali suttas, the Buddha did not mention the nimitta because he only mentioned ekaggatā.
Nimittas arise, and if your intent is not the continuity of this nimitta, another nimitta will arise in its place. In meditation we try to keep to one nimitta. “Uppādo” means arising. When anusaya (temptations) arise due to āsava (cravings), we need to stop that temptation and be at “pavattam” (continuity of it or go with it).
If we go along with the unbroken and consistent maintaining (“pavattaṃ”), then it becomes a nimitta (literally a “sign”). A nimitta is a characteristic that is associated with a particular act. For example, for an alcoholic a picture of an alcohol bottle or a bar (or where one normally drinks), or even seeing a friend with whom one drinks often, can be a nimitta; when any of such a “symbol” comes to the mind, it reminds of the drinking act and gets one in the “mood”. Now you will see that it is not so difficult to ignore a nimitta.
Go back to your object of meditation. Don't pay attention to nimitta, just let go of it. if you hold nimitta, you'll become crazy in no time. Trust me.