Green’s Dictionary of Slang

1. (Aus.) a long time, or something that appears to be.

1936

(con. 1830s–60s) ‘Miles Franklin’ All That Swagger 147: Sure, I have a cold as long as a wet week. Wan day more won’t hurt Bella either way.

2. (Irish, also wet day) a short time [? being wet it ‘shrinks’].

1956		B. Behan Quare Fellow (1960) Act II: It’ll break that young screw up, and him only a wet day in the place.
1997		Irish Times 21 May n.p.: Sure, women have only been out of the kitchen a wet week if you think about it [BS].