Free spin bonuses are usually restricted to a particular slot game or group of slot games chosen by the online casino. Slot games vary in terms of payout potential. This is expressed by RTP (return to player) and volatility.
A slot with an RTP of 97% would distribute 97% of stakes as winnings determined by the slot’s volatility. High volatility slots pay out larger less frequent wins. Conversely, low volatility slots pay out smaller wins more frequently.
Combined, these two metrics provide an indication of the payout potential of a slot game. In terms of your 30 free spins with no deposit bonus, a smart move would be to play high RTP slots with low volatility for the best chance of getting enough small wins to meet wagering requirements.