Is this true? How much time would it take to deliver on emergency basis?
If it's a rolling dry-lease, immediately. We pay, they pull out RuAF aircraft, strip it of any classified secret sauce it may have, attempt to augment some interoperability with Indian systems, transplant IAF user interface to the best they can, and hand them over immediately. These aircraft will be good enough for use within formations, or even visual range engagements. Knowing IAF, these will be positioned more toward shoring up air-defences against Pakistan.
Eventually UAC will begin manufacturing these aircraft, and then India will have the option to acquire those aircraft, or keep the leased aircraft and begin retrofitting them up to MKI spec.
If we're dry-leasing, we'll very likely get RuAF-spec Su-30SM
As for MiG-29, it's anyone's guess. They closest thing they have to 29UPG is 29SMT, but they only have like 22 in their entire inventory.
Again both leased aircraft will only be good for fattening up formations or for use as reserves. They'll have 99% of the kinetic capabilities, and if you're using them to lob R-73/77, you're fine.
Just imagine if MMRCA clicked in 2009 and we bought F-18 Super Hornet. One phonecall to DC, and they'd deliver from their inventory the next day.