Briefly? We're never safe

Gurnall expounds another time — or a pair of times — which Satan finds advantageous for the attack:

Fifthly, After great manifestations of God’s love, then the tempter comes. Such is the weak constitution of grace, that it can neither well bear smiles nor frowns from God without a snare; as one said of our English nation, Totam nec pati potest libertatem nec servitutem; it cannot well bear liberty nor bondage in the height: so neither can the soul; if God smile and open himself a little familiarly to us, then we are prone to grow high and wanton; if he frown, then we sink as much in our faith; thus the one, like fair weather and warm gleams, brings up the weeds of corruption; and the other, like sharp frosts, nips and even kills the flowers of grace. The Christian is in danger on both hands, therefore Satan takes this advantage, when the Christian is flush of comfort, even as a cheater, who strikes in with some young heir, when he hath newly received his rents, and never leaves till he hath eased him of his money; thus Satan lies upon the catch, then to inveigle a saint into one sin or other, which he knows will soon leak out his joy. Had ever any a larger testimony from heaven than Peter, Matt. 16:17; where Christ pronounceth him blessed, and puts a singular honour upon him, making him the representative for all his saints? No doubt this favour to Peter stirred up the envious spirit sooner to fall upon him. If Joseph’s party-coloured coat made the patriarchs to plot against him, their brother, no wonder malice should prompt Satan to show his spite, where Christ had set such a mark of love and honour; and therefore we find him soon at Peter’s elbow, making him his instrument to tempt his Master, who soon espied his cloven foot, and rebukes Peter with a ‘Get thee behind me, Satan.’ He that seemed a rock even now, through Satan’s policy, is laid a stone of offence for Christ to stumble at. So David, when he had received such wonderful mercies, settled in his throne with the ruin of his enemies, yea, pardoned for his bloody sin, now ready to lay down his head with peace in the dust; Satan steps in to cloud his clear evening, and tempts him to number the people; so ambitious is Satan, then chiefly, to throw the saint into the mire of sin, when his coat is cleanest.

[William Gurnall and John Campbell, The Christian in Complete Armour (London: Thomas Tegg, 1845), 48.]

So to the question "When may I safely lay down my guard and take off my armor?", the answer would be, "Never in this life."