OSPF Hello and Dead Timers

A router configured with OSPF sends out Hello Packets every 'x' seconds to maintain connectivity with its neighbors. The time elapsed between 2 hello packets can be manually configured and is called the Hello Interval.
The amount of time elapsed before a neighbor will declare a router 'down', after the neighbor stops hearing Hello Packets is the Dead Interval.

By default the Hello Interval to Dead Interval ratio is set as 1:4 within the protocol itself.

Depending of the type of network being used, the timers vary.
For a Broadcast and Point to Point network, the Hello Interval is 10 seconds and the Dead Interval is 40 seconds.
For other types of networks (Non-Broadcast, Point-to-Multipoint and Point-to-Multipoint Non-Broadcast), the Hello Interval is 30 seconds and the Dead Interval is 120 seconds.