The answer depends upon your appetite for risk and where your Sats are now.

If on an exchange that number is very very low and dependant on the exchange withdrawal fees. Lowest viable amount (withdrawal fees determine viability)

If Sats are in a custodial hot wallet on your phone such as Wallet of Satoshi, the answer is equal to the amount of fiat cash you’d be comfortable carrying in your purse/wallet.

If Sats are in a non custodial hot wallet such as Blue Wallet, the answer depends on the cost of your cold storage option. For example if you have US$100 worth of Sats and your hardware wallet was going to cost US$100, then I would suggest that is not worth the precaution. However if you have US$10,000 = approx 30,000,000 Sats then that is likely worth the cost of the hardware wallet to secure it.