What is the duration of Jhana? What is the duration of Jhana?
How long the duration of each Jhana.
For instance, what is the duration of the first Jhana?
Say if I have first Jhana only for one minute, can I say that I was in the first Jhana?
After the absorption, the practitioner can say he is the obtainer of the first Jhana if the five hindrances still not arise instead of Jhana practice.
If that obtainer can't absorpt the first Jhana, but he has not an arising of the five hindrances, we still can call him as the obtainer of the first Jhana because he is in access concentration state, Upacāra-Jhāna, of the absorption concentration state, Appanā-Jhāna, which certainly going to arise if he still can keep the access concentration go on continuously.
The five hindrances are a major one important keyword of this question. Focus on it.

At the first time absorption, Jhana arises only one moment, but it is going to take the longer duration when the practitioner practice that Jhana to be the mastery skillful expert, Vasī.
It's like when the practitioner practices the weight training, no one can hold up the weight in long duration, but the practitioner can hold the weight up longer and longer after training the weight more and more.
The path of purification, pathavikasinaniddesa:

Sā ca pana ekacittakkhaṇikāyeva.
Ñ(IV,78): But that (first time of the first Jhana) absorption is only of a single conscious moment.
Sattasu hi ṭhānesu addhānaparicchedo nāma natthi paṭhamappanāyaṃ, lokiyābhiññāsu, catūsu maggesu, maggānantaraphale, rūpārūpabhavesu bhavaṅgajjhāne, nirodhassa paccaye nevasaññānāsaññāyatane, nirodhā vuṭṭhahantassa phalasamāpattiyanti.
Ñ: For there are seven instances in which the normal extent [of the cognitive series] does not apply. They are in the cases of the first absorption, the mundane kinds of direct-knowledge, the four paths, fruition next after the path, life-continuum jhāna in the fine-material and immaterial kinds of becoming, the base consisting of neither perception nor non-perception as condition for cessation [of perception and feeling], and the fruition attainment in one emerging from cessation.

The path of purification, Vasī:


Tatrimā pañca vasiyo āvajjanavasī, samāpajjanavasī, adhiṭṭhānavasī, vuṭṭhānavasī, paccavekkhaṇavasīti.

Ñ: Herein, these are the five kinds of mastery: mastery in adverting, mastery in attaining, mastery in resolving (steadying the duration), mastery in emerging, and mastery in reviewing.
... (See the explanation inside the link and book)...