Saudi estimates on time to restart will be available in a few days. so this estimate is an exercise.  A 1960 article from “Aramco World”, titled Sweetening Up the Crude, is relevant. When we think of a refinery, the “cracking unit” comes to mind, as it is most central to producing refined petroleum products. But crackers are not central to the main job of Abqaiq, which is removing sulfur, and dissolved flammable gases under high pressure. Quoting,

All the crude produced in Saudi Arabia—except for that of the offshore Safaniya Field in the Persian Gulf—is "sour." At ground level the pressure may be as high as 1,000 pounds per square inch (or "1,000 psi," as the engineers say). It must be reduced considerably before it reaches the stabilizer, so it's sent first to a gas-oil separator plant, or "GOSP." There are eleven of these in the Abqaiq area.

Now, the gas can't be allowed to "blow off" all at once. If it did, a considerable amount of liquid would also be lost—something like shaking a bottle of soda pop before uncapping it.

The gas is released in stages in a series of drums or columns, known as separators, before it reaches a spheroid where the pressure is cut down to about 2 psi. By now, most of the light hydrocarbon gases have been removed, but the gas is still "sour." The next step is to pump it to the sour-crude storage tanks at the stabilizer to await processing.

If water and sulfur are both present in crude, sulfuric acid forms, which eats through low alloy steels in common use.

The  inlet pressure of the (as of 1960) 11 GOSPs  is about 1000 psi, 10X the operating pressure of a typical “cracker” The GSOP’s at Abqaiq are made of heavy gauge stainless steel to resist both pressure and corrosion by sulfuric acid.   This is required because there is no control over the  composition of the oil as it comes out of the ground. Later in processing, combinations of stabilizing chemicals, and removal of water, make possible (Science Direct) transport of some sour crudes  in HSLA steel pipelines.  But at Abqaiq, Aramco actually transforms sour into sweet, with a resulting price premium.

• The drone warheads are small, too small to destroy a substantial building. But if a drone hit a GOSP, the high operating pressures and  abundance of flammable gases would multiply the effect, resulting in total destruction.
• The requirements of replacement: thick stainless steel, custom fabrication, and flawless welding. These days, stainless production and fabrication is principally done in China, a supply chain risk.
• Other infrastructure, such as pumps and automation, are potentially serviceable with spare parts. This is not the case of the GOSPs.
• This is not a simple Shukhov thermal cracker, of the kind the massive WWII Allied bombing raids on Ploiești found so hard to flatten. High technology and large scale exact the penalty of long lead times.

The estimate,  purely as an exercise, is 120 days for partial resumption of lost production.

To say there is no defense against this level of attack is incorrect. The AN/SEQ-3 Laser Weapon System is a good fit, though study of saturation attack is a requirement. Since,

• The targets are large, high value infrastructure,
• Large areas of Houthi control, and all of Iran, enjoy a shield from direct combat conditions,

Saturation attacks are more likely than if the origin is  a combat zone.