Its usually 1-2 or so months without a post which classes a topic to be gravedig-able. With the exception of pinned topics, forum game topics, or unless you have a really good reason to bump it.
Someone correct me about the dates, because I'm not 100% sure on them.
I was pretty sure it was 120 days, because after 120 days you get a notification thing up the top saying "This topic is 120 days old or older are you sure you want to post". If I'm correct 120 days is around 3 and a half to 4 months. But I'd say grave digging in this community is usually 1 month - 2 months so your right Luke. <3