You don't HAVE to spend anything at all.
Many of my friends don't.
Get a free trial membership; then there are places that GIVE away quite good quality stuff - search inworld for Warchild, for instance. Or you can camp (that's where the time comes in) for a few linden dollars (I am afraid I buy a few RL dollars worth at a time. Camping is boring and I love shopping
and you can buy stuff for 1L$ at various "newbie friendy places". You need real $$ mostly to get land.