I don't think anyone is alive that can answer that from direct personal knowledge.
A quick search yielded an interesting article which answers some of our questions:
Oxygen Toxicity [extra info added by myself]:
'In normal humans the first signs of
[pulmonary] toxicity appear after about 10 hours of oxygen at 1ATA. Clinical features can be divided into three phases (a) Tracheobronchitis [
a sudden or longterm inflammation of the trachea and bronchial airways,](b) ARDS [
respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin coloration (cyanosis). For those who survive, a decreased quality of life is common,](c) Pulmonary interstitial fibrosis [
inflammation and scarring that make it hard for the lungs to get enough oxygen.]
100% oxygen can be tolerated at sea level for about 24–48 hours without any serious tissue damage. Longer exposures produce definite tissue injury. Oxygen at 2 ATA produces characteristic pulmonary signs and symptoms beginning with mild carinal irritation on deep inspiration 3–6 hours into the exposure, intense carinal irritation an uncontrolled cough after about 10 hours and finally chest pain and dyspnoea. Symptoms subside 4 hours after cessation of exposure in majority of patients.'
And further down we have quantifying information:
'Unit of Pulmonary Toxicity Dosage (UPTD) is a theoretical concept used to predict the extent of pulmonary damage with complicated and prolonged therapy. One minute of 100% oxygen at 1 atmosphere is taken to produce 1 UPTD. A UPTD of 1425 will produce a 10% reduction in the vital capacity.'
Now that sounds familiar - UPTD is used by Bsac 88 Nitrox tables in the exact same ratio - 1 UPTD = 1 min of 1.00 PO2.
The 10% reduction in vital capacity is referenced to Edmonds C, Lowry C, Pennefather J, editors. 'Diving and Subaquatic Medicine' Butterworth-Heinemann; Oxford: 1992 book which I do not have access to. I am therefore unable to verify with the source if the said reduction is temporary or permanent, and is it theoretical (as UPTD) or based on actual findings?
However, jumping to another book (Jain, K.K. (2017). Oxygen Toxicity. In: Textbook of Hyperbaric Medicine. Springer, Cham.;
https://doi.org/10.1007/978-3-319-47140-2_6) yields this observation:
'The concept of a “unit pulmonary toxic dose” (UPTD) was developed to enable comparison of the pulmonary effects of various treatment schedules of HBO (Bardin and Lambertsen
1970). The UPTD was designed to express any pulmonary toxic dose in terms of an equivalent exposure to oxygen at 1 ATA. It is only an arbitrary measure and did not allow for the recovery between HBO exposures. For example, 10 HBO treatments at 2.4 ATA for 90 min each would give the patient a UPTD of >200 and would indicate significant pulmonary toxicity with a 20 % reduction in vital capacity. In practice, however, no clinical evidence of pulmonary toxicity is seen with this schedule. There is no significant impairment of pulmonary diffusing capacity in divers who have been intermittently exposed to HBO at 4 ATA for years. The term “UPTD” is now hardly used in clinical practice of HBO therapy.'
It does look like the UPTD limits are on the conservative side. NOAA's maximum single oxygen exposure for pO2 1.00 is 300 mins = 300 UPTD. Based on the Elsevier article above, the first mild pulmonary symptoms (irritation) should appear at 360 to 720 UPTD, with more serious symptoms including cough at around 2000 UPTD.
