There are a couple factors to determine when to replace your roof. One thing is how old is the existing roof. Depending on when it was installed the asphalt in the shingles could be hardening and shrinking causing curling/cupping. Another is if you are experiencing any leaks. Finally are there missing or damaged shingles. If the roof is old or poor quality you could have loose shingles that blow off in high winds. If you experience any or all of these you should consider having Summit Roofing inspect the roof to determine replacement
It all depends on the type of shingle installed and the manufacturer. A 3 tab builders grade roof generally lasts anywhere from 14-17 years in our area and an architectural shingle generally last at least 25-30 years in our area. This all depends on the quality of the manufacturer and the quality of the installation.
An asphalt roof can be installed all year round, However the ideal time would be in warmer temperatures such as spring and fall.