The general consensus is, hot enough to take advantage of the digital resolution available while not so hot that the signal clips or you have problems later, during mixdown. For dynamic tracks, the recommended level — offering a good compromise between resolution, strong output, and headroom — is between -18dB and -12dB. This allows plenty of headroom for transients, dynamics, and level spikes, and is low enough that there’s room to boost EQ, add level during mixing, and so on. but it’s still hot enough to provide good resolution and level.
For certain signals, such as very dynamic drums, this might still be too hot. But in general, -18 to -12dB makes a good starting target.