Basically, if notice period is 1 month, then you only need to serve 1 mth. Any longer "notice" is really dependent on how much you like your current firm and partners. Remember once you tender you're the "guy that's leaving" (i.e. no good opportunities will be given to you, maybe more work thrown to you to deal with). If you got good relation with the boss, don't want them to be sad or short-staffed, by all means go tell them abit earlier. But if your boss is the vindictive type or the type that likes to immediately tie up loose ends (i.e. you tell them you want leave Jan, but he fire you immediately type), then maybe hold off on giving notice?

If the company only asks for 1 month notice, that means that they believe you are replaceable in 1 month, so I wouldn't give additional notice unless I am sure it won't come back to bite me in the ass.

Generally, SLB responds faster (1-2 weeks), DRD response rate fluctuates.