How long did Silver Surfer arrive at a planet before Galactus would arrive to consume it? Would they have enough time to prepare or leave or cover themselves in ranch dressing?
How soon Galactus arrives at a designated planet depends on where he was in his energy gathering cycle.
Factors such as:
- When he last ate, and if he was being starved by his herald or trying to resist eating worlds;
- whether the last planet was sufficiently rich in energy;
- how much energy Galactus has used since he has last consumed a planet. If he spent energy in battles against cosmic-level foes, he would deplete his energies sooner.
In the early days of his planet consumption, the Silver Surfer had months to find worlds, (by his standards) preferably without sentient life on them.
As the intervals between planets grew shorter (no we are never privy to what the intervals were specifically), it became harder to find planets that didn't harbor sentient lifeforms.
The Surfer became so distraught feeding Galactus worlds with sentient life on them that Galactus was forced to eventually mindwipe the Surfer any time sentients were killed so that he would continue his duties as herald.
The Surfer would eventually bring Galactus to planets with high levels of technological capacity so that they would either have the capacity to drive Galactus away or have the ability to evacuate their people from their planet.
Yes, sufficiently advanced races often did manage to evacuate some measures of their planet's populations and a number of them gathered together hoping to exact revenge on Galactus in the future.
This eventually led to Galactus being led to Earth and the Surfer rebelling and attacking Galactus directly. It was estimated by the time Galactus got to Earth, he was needing a planet every few months.
As for the speed of his arrival, Galactus had been known to appear in minutes once he was summoned by his herald, if his hunger was great enough. Evidently the Power Cosmic was capable of making connections to its master at faster-than-light speeds.