A good rule of thumb is that there are approximately two years between funding rounds. While this could potentially include the gap from Series C to IPO, some companies will raise additional funding in Series D or even Series E rounds deciding to undergo a liquidity event. Raising more rounds can help you achieve the valuation you want ahead of the IPO and also time your liquidity event properly. The timing of IPO is heavily dependent on market dynamics when you feel your business is ready to operate as a public company.