Can an 8V battery charge a rechargeable sealed lead acid 6V battery? How long will it take to fully charge?

Charging the six volt battery is by the current flowing into it. If it is rated at 7Ah (ampere hours) capacity, that means it can supply 0.7 amps for 10 hours usually, because the rating applies particularly to 10 hours (usually). If the battery needs charging completely, the usual approach is to charge at the 10 hour rate (0.7A) for 14 hours. If it is charged at a higher current or lower current it charges proportionally more quickly or slowly than that 14 hours, within reason. However overheating can be an issue with faster charge. Some batteries are not to exceed 40C for example. With very low currents it might never charge, as its own self discharge could exceed the charge. In general an 8 volt battery can charge a 6V battery. For a 3 cell lead acid battery it requires around 7V to fully charge. This voltage is required towards the end of the charge as the battery voltage increases. This has about 1V difference, which should be sufficient, depending on voltage drops through the charger and the connections to obtain reasonable charge currents. The charger has the role of keeping the current constant throughout the charge at an appropriate value and stopping the charge at an appropriate point. There are various strategies that might be used. The source (redox battery) needs to supply the appropriate current for the time involved and maintain sufficient voltage for this current to flow into the battery through the charger. Thus it needs to supply 8V at about 0.7A for 14 hours to charge a 7Ah battery. A suitable charger is needed to limit the current. This could be a resistor in its most primitive form. Calculating, use 8.4V to 5V worst case voltage difference, so for the example battery: R = 3.4V/0.7A = 4.86 ohms at 5W rating, wasting 2.38W. The current could drop as low as 0.2 amps with this though, as the difference can be only 1V. The charge can be stopped after a predetermined time, or for lead acid batteries when the voltage across the battery reaches almost 7V. If the redox battery cannot supply enough current, recalculate with a current it can supply, which of course will take longer.

Yes. That is the only way to charge it! Supply voltage (in your case 8 V) must be greater than battery voltage (6 V). How long it takes to fully charge the battery? It depends on characteristics of supply battery / charger and the battery itself. Charging current is calculated by difference in voltage divided by total internal resistance of supply and battery added together.(Vs-Vb) / (Rs+Rb).