Per ECJ what is the limiting factor on UK unilaterally revoking A.50 notification?
expiry date of WA

No. The expiry date is two years after the A.50 notification has been issued, or the date of the entry into force of the Withdrawal Agreement, whichever is the sooner. Both mark the end of the UK's formal membership of the EU, and the end of any possibility of revoking an A.50 notification.

Article 50.3:

3.   The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period."