How soon must the under-layer be covered with shingles?
The length of time isn't what's important here, it's the shape the under layer is in that counts.
As long as the under layer is in good condition, without bumps and ripples that will distort the shingles, you should not have to replace the under layer. Try to take a look before shingles are installed to make sure your contractor has a smooth and level surface before he starts working.
For more information on shingle installment speak with Asonic Roofing Calgary.