ISC DHCP infinite lease time

It is not mentioned explicitly in the manpage, but setting lease time to -1 in any of the options you mention,

default-lease-time -1;
max-lease-time -1;

is effectively disabling the expiry time of the leases, so their expiration will be effectively set "to infinity".

You should not configure an infinite lease time. The reason of having DHCP is to have a central management and flexibility. Making the lease time infinite, you will kill the flexibility.

I would suggest specifying lease times in seconds. So where:

default-lease-time 600; This being ten minutes
max-lease-time 7200; This being two hours

try:

default-lease-time 86400; This being one day
max-lease-time 604800; This being one week

You could try 2592000 which is 30 days.

I wouldn't exceed that.