Kind of a how long’s a piece of string question, as depends on the rain strength, direction, etc.
I put strips of 450mm wide DPM plastic over my cavity wall (held down with bricks) and that gives it protection
I’ve laid bricks in the light rain, and been caught out and had sudden downpours as I’m laying.
I think providing the mortar is dry enough and/or the rain gentle enough to not wash out the joints you’ll be fine as long as the top is covered when it does rain.
I put strips of 450mm wide DPM plastic over my cavity wall (held down with bricks) and that gives it protection
I’ve laid bricks in the light rain, and been caught out and had sudden downpours as I’m laying.
I think providing the mortar is dry enough and/or the rain gentle enough to not wash out the joints you’ll be fine as long as the top is covered when it does rain.