Reciprocity failure, without the maths

What happens

At normal shutter speeds, halving the light and doubling the time cancel out. Below roughly one second that stops holding: film becomes progressively less efficient, so a metered ten seconds may need thirty or more.

Rules of thumb

Every film behaves differently, and the manufacturer's table beats any general rule. Absent a table, a metered 1 second often wants ~1.5, a metered 10 seconds wants ~30, and a metered 60 seconds can want several minutes.

Contrast moves too

Highlights keep building while shadows stall, so long exposures gain contrast as well as density. If you develop normally, expect harder negatives than you metered for.

The practical approach

Bracket. Long exposures are cheap in frames and expensive in time — shoot two or three and choose later.