Speaking from experience, a first generation Pencil will optimally charge at its fastest rate when connected directly to your iPad. From fully discharged, a healthy Pencil battery will achieve full charge in 15-20 minutes. A few seconds charging from the iPad will provide about 15 minutes of active use.
When charging from a Power Adapter, using the adapter supplied with the Pencil, charging of the Pencil will take longer. This method of charging, whilst viable, appears to be intended to “float charge” the Pencil, over longer periods, to maintain its charge. Whilst not having explicitly measured charging times from a Power Adapter, the charging time from a low charge state is much longer - possibly up to a few hours.
As a side-note, it is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless.
If the Pencil Battery fails, the only remedy is to replace the Pencil.
