Raspberry Pi Forums

When I plug my Pi in and boot up, I am able to connect over the air with SSH at 192.168.1.100 (static IP address set in /etc/network/interfaces).

However if I boot up my Pi and leave it for 1-2 hours or longer, when trying to connect to my Pi with SSH at the same address, I always get the SSH error

It seems as though the WiFi may shut down on the Pi after no useage for a certain period, however looking at the Pi physically, the lights seem the same as they are when directly booting up - Flashing WiFi Usb and other solid PI board LEDS.

Some usb wifi adaptors have a power-saving mode whose operation would have the effect you are describing. You may need to research how to disable this feature for your particular model of wifi adaptor. (The specific details vary from one model to another.)

If you're using Putty, you'll find it will time-out after a certain period of inactivity. You can then simply restart the session.
Alternatively there is a putty setting to "keep-alive"

edited to say: timeout after a period of inactivity is normal behaviour for ssh servers.