How long does it take for an anime to be subbed?

It depends on the subbing company/people. Some anime takes just 1-2 hours to sub, like One Piece or Sword Art Online. Other animes take longer to sub usually one week like Detective Conan.

When its a TV series there's an episode released every week and the scanlation group has to sub various animes so they have to keep on track by trying to sub within the 24 hour period but an OVA is just a one off episode so its the last of their priority so waiting a month and a half is very possible.

TV series : Subbed mostly under 24 hours.

OVA: 1 or 2 months.

Movie: 2 to 3 months after DVD release.

Different subtitle companies take different amount of time. As said on the One Piece wiki:

Types of Fansubbing

The following types of fansubs are not totally separated, e.g. a video can have a speed sub, which is partly hardsubbed (e.g. for karoke, special effects, fansub group information) and partly softsubbed (e.g. for normal dialogues).

Speed Sub

Speed subs are fansub releases that are released quickly after the episode is originally aired. Due to the speed at which these are released, they typically have not had as much editing done to the script, and they often times have less detailed effects. However, the episodes can be released even the day the raw file is available, making these preferable to some people, although others do not care for speed subs because of the lack of quality typically seen in them. For One Piece, ADC-Elites is a speed sub.

Hardsub

With a hardsub, the subtitles are encoded into the video file itself, resulting in one file in which the subtitles cannot be removed or altered in any reasonable way. Fansub groups typically use this format to protect there translations, timing and styling from being "stolen" - though it does not mean that they can be assured that their work (mostly the translation, but the timing and effects as well) is not being taken - and because of playback reasons in case of users with old PCs or special technically inferior playback devices. For One Piece, Nakama-fansubs, Kaizoku-Fansubs, ADC-Elites (along with many other groups) are hardsubs.

Softsub

Unlike hardsubs, softsubs are not irreversibly merged with the image but rendered onto the video during playback. They may be packed inside a mkv container together with video, audio and necessary fonts for playback or come as a separate file. With a softsub, the user can modify how the subtitles appear on the video, as well as being able to modify the script itself. For example, Yibis uses softsubs.

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A normal subtitling takes maybe up to a week. You've certainly waited quite a while for this subtitle. Perhaps a single person was working on the whole thing.

You mean fansubs, yes? If it's fresh and popular, it wouldn't take a day to have it speedsubbed from the time of it's airing. (ToS doesn't seem like that type of anime. Not to mention it's an OVA, yeowch, that's gonna take longer!) What gets an anime fansubbed is a group that's interested in it. If there is one (hopefully, there is, eh?), they're probably still translating. Quality subbers take longer due to various reasons especially (but not limited to) quality checks. If you're lucky, it won't be long. If not..

In modern times, at least for popular shows, it usually doesn't take more than a couple days after the episode airs for the fansubs to start appearing. The Internet has greatly toughened the competition for releasing subs as soon as possible.

usually a 24hours to a week