Why did Noah Vosen lie to Nicky Parsons, that backup was arriving approximately ...

In response to Vosen’s ID challenge, Nicky Parsons gave her “response if normal”, but he then lies to her and tells her that the backup will be an hour when he knows that they're only minutes away.

Why did Noah Vosen fib to Nicky?

.

Vosen: Nicky, I need to do an ID challenge. Code in... Sparrow.

Nicky: Response... Everest.

[snip]

Vosen: [with Nicky on hold] How long until backup? [back on speaker] Nicky, I need you to stay put and secure the premises. Backup will be arriving in approximately one hour. Do you copy?

Nicky: Copy, sir.

(PHONE BEEPING)

Bourne: How long do I have?

Nicky: Three minutes.

Per the script, Landy and Vosen strongly suspect that Bourne is in the room from the outset of the call. Not only is there the coincidence that a Treadstone handler has just shown up in the middle of an investigation into Jason Bourne, but Nicky’s weird and evasive responses all but confirm it for them, most notably when she doesn’t immediately volunteer the information that she has discovered two dead CIA operatives on-site(!), only revealing it when she is specifically asked.

NICKY (V.O.): This is Nicky Parsons. Who’s this?

Landy reacts to her voice...

Both Vosen and Landy strongly suspect Bourne is there and listening...

Vosen thinks that telling her (and by extension, telling Bourne) that their agents will be there "in three minutes" will immediately alert Bourne to run away as fast as possible. If he says that they're "an hour away", there's a slim possibility that their target will still be present when the backup agents arrive at the safe house.

You might wish to note that Landy appears contemptuous of Vosen's feeble attempt at subterfuge.

VOSEN: How long till back up arrives.

Wills holds up three fingers. Vosen unmutes.

VOSEN (CONT’D) (into microphone again) Listen, Nicky. I want you to stay put and secure the premises. It could be an hour until another team arrives.

LANDY piercing VOSEN with a look...

and

NICKY: They know you’re here.

BOURNE: How long do I have?

NICKY: Three minutes...