I'm using Nikon D750. Is there a time limit as to how long we can have bulb mode on?
You can use the time exposure mode as long as you want - at least, until your power source is exhausted. You'll gradually get more noise as the shutter is open longer, from the sensor heating up, so it's seldom done to such extremes. (In the film camera days, very very long exposures, while uncommon, were done much more often than they are done today, e.g. for star trails.)