<human>: Answer by taking a quote from the following article:

Led Zeppelin is the eponymous debut studio album by English rock band Led Zeppelin. It was first released on 12 January 1969 in the United States and on 31 March 1969 in the United Kingdom by Atlantic Records. Featuring integral contributions from each of the group's four members, the album was recorded in October 1968 at Olympic Studios in London, and established their fusion style of both blues and rock music. It also attracted a large and devoted following to the band; Zeppelin's take on the emerging hard rock sound endeared them to parts of the counterculture on both sides of the Atlantic Ocean.

In a 1990 interview, Page said that the album took only about 36 hours of studio time (over a span of a few weeks) to create (including mixing), adding that he knew this because of the amount charged on the studio bill. One of the primary reasons for the short recording time was that the material selected for the album had been well-rehearsed and pre-arranged by the band on Led Zeppelin's tour of Scandinavia in September 1968. As Page explained, "[the band] had begun developing the arrangements on the Scandinavian tour and I knew what sound I was looking for. It just came together incredibly quickly."  In addition, since the band had not yet signed their deal with Atlantic Records, Page and Zeppelin's manager Peter Grant paid for the sessions entirely themselves, meaning there was no record company money to waste on excessive studio time. In another interview, Page revealed that the self-funding was to ensure artistic freedom: "I wanted artistic control in a vise grip, because I knew exactly what I wanted to do with these fellows. In fact, I financed and completely recorded the first album before going to Atlantic ... It wasn't your typical story where you get an advance to make an album - we arrived at Atlantic with tapes in hand ... Atlantic's reaction was very positive - I mean they signed us, didn't they?"  The group reportedly recorded their songs for PS1,782. Led Zeppelin archivist Dave Lewis noted that "[w]ith the possible exception of the 12 hours that the Beatles took to record their first album at Abbey Road, rarely has studio time been used so economically. Led Zeppelin's debut album went on to gross more than PS3.5 million, just short of 2,000 times more than they invested!"  For the recordings, Page played a psychedelically painted Fender Telecaster, a gift from Jeff Beck after Page recommended his boyhood friend to the Yardbirds in 1965 as a potential replacement for Eric Clapton on lead guitar. This was a different guitar from those he favoured for later albums (most notably a Gibson Les Paul). Page played the Telecaster through a Supro amplifier. He also used a Gibson J-200, borrowed from Big Jim Sullivan, for the album's acoustic tracks. For "Your Time Is Gonna Come" he used an out-of-tune Fender 10-string steel guitar.  Two other songs from the sessions, "Baby Come on Home" and "Sugar Mama" both appeared on the 2015 Coda reissue (although "Baby Come on Home" was first released on the 1993 compilation Boxed Set 2.) CANNOTANSWER

Why did it only take 36 hours?
<bot>: material selected for the album had been well-rehearsed