dnd-5edurationmetamagicsorcererspellcasting
Can you use the sorcerer's Metamagic option Extended Spell more than once on a single spell? In other words, can you pay more SP to extend the duration by one factor?
For example:
Say a given spell has a duration of 1 minute. If you spend 1 Sorcery Point, the duration changes to 10 minutes; spending 3 SP changes the duration to 1 hour; 4 SP makes it 8 Hours; 5 SP makes it 24 hours. Does this work?
While skoormit has pointed out that the average percent increase for 1 die is 21.43, the overall increase for multiple dice is not straightforward, and highly depends on your Charisma modifier, more so for more dice.
I used a small python script to roll xd6. The lowest c dice (where c = positive Cha modifier) were re-rolled, unless there were less than c dice below average. The following data was gathered by averaging over 100,000 of such sets for each combination of x and c.
It shows the percentile increase of the average roll, i.e. ((new avg / old avg) − 1) × 100.
As you can see, the increase drops rapidly below 20% for x > c, though the slope becomes less steep as c increases.
The errata for the PHB has clarified the restriction further from the original printing:
To be eligible for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell’s current level.
By default, eldritch blast does not have a range of self and is capable of targeting only one creature. It is thus eligible to be twinned.
However, the spell becomes capable of targeting more creatures once the caster reaches level 5:
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
Thus, once the caster reaches 5th level and above, they can no longer twin eldritch blast.
Note that eldritch blast (and all other similar cantrips) scale with character level not class level as confirmed (unofficially) by Jeremy Crawford on Twitter. (See Do Cantrips use your character level or class level? for more discussion about that)
Best Answer
No, you can't. In general, you can only use one metamagic, once, on any spell.
The only such note is this one, on Empowered Spell:
In particular, Extended Spell has no such note.
So no, you can only Extend a spell once. Also, Extended Spell doesn't work the way you seem to think it does. All it does is double the duration, not increase the duration by "factors".